Daily Mishnah · Expert – Beit Midrash Analysis · Standard
Mishnah Kelim 13:4-5
Sugya Map & Snapshot
The Issue
The core of Mishnah Kelim 13:4-5 is the ontological status of shiyurei kelim (the remnants of damaged utensils) and the mechanics of yichud (functional designation). We are forced to confront a foundational question in the metaphysics of tumah: Does a vessel’s susceptibility to impurity (kabbalat tumah) depend on its original, holistic form (shem kli), or does it persist so long as any residual, localized utility (ra'uy l'melekhet yichud) remains?
┌───────────────────────────┐
│ UTENSIL DESTRUCTION │
└─────────────┬─────────────┘
│
Is the structural form ruined?
│
┌───────────────┴───────────────┐
▼ ▼
[ YES ] [ NO ]
│ │
Does a minor, alternative Does the original primary
utility still remain? function still persist?
│ │
┌───────┴───────┐ ┌───────┴───────┐
▼ ▼ ▼ ▼
[ YES ] [ NO ] [ YES ] [ NO ]
│ │ │ │
Is there a need TAHOR TAMEI TAHOR
for "Yichud"? (No Utility) (Intact Vessel) (Ruined Form)
Nafka Minot
- The Status of Split Vessels (Nihleku l'Shenayim): If a tool is split in two, do both halves retain impurity because each can perform a fraction of the original labor, or are they tahor because the unified shem kli has been shattered?
- Composite Utensils (Yad v'Tafel): Does the auxiliary component of a vessel (e.g., a wooden handle serving a metal blade) determine the vessel's classification, or do we follow the dominant material?
- The Mechanics of Substituted Utility: Can a broken tool transition from its primary category to a secondary category without physical modification (shinui ma'aseh), relying solely on mental designation (haskamah)?
Primary Sources
- Mishnah Kelim 13:4
- Mishnah Kelim 13:5
- Bava Metzia 82b
- Shabbat 60a
- Chulin 124b
Text Snapshot
The Mishnah in Mishnah Kelim 13:4 states:
"הַמַּעֲצָד וְהָאִזְמֵל וְהַמִּפְסֶלֶת וְהַמַּקְדֵּחַ שֶׁנִּפְגְּמוּ – טְמֵאִים. נִטַּל חִסּוּמָן – טְהוֹרִים. וְכֻלָּם שֶׁנֶּחֶלְקוּ לִשְׁנַיִם – טְמֵאִים, חוּץ מִן הַמַּקְדֵּחַ."
In Mishnah Kelim 13:5, we read:
"מַחַט שֶׁנִּטַּל קֻפָּהּ אוֹ חֻדָּהּ – טְהוֹרָה. אִם הִתְקִינָהּ לְמַלְתֵּחַ – טְמֵאָה. סַקְרִין שֶׁנִּטַּל קֻפָּהּ – טְמֵאָה, מִפְּנֵי שֶׁכּוֹתֵב בָּהּ..."
The dikduk of "נִטַּל חִסּוּמָן" (its steel edge was removed) demands precision. The chisum is not merely the tip; it is the high-carbon steel cladding welded to the soft iron core. When this cladding is lost, the tool remains physically intact but functionally soft. The linguistic root of chisum derives from Deuteronomy 25:4: "לֹא תַחְסֹם שׁוֹר בְּדִישׁוֹ" (do not muzzle an ox)—meaning to shut, seal, or harden. Without this tempering, the tool loses its structural resilience.
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Readings
Rambam: Metallurgy, Form, and Structural Essentialism
To understand the Rambam’s reading of Mishnah Kelim 13:4, we must analyze his commentary on chisum. The Rambam writes:
"וקצה הכלי לבד אשר בו יחתכו או יקבו או יחליקו הוא מברזל נקרא אצייר"י בלע"ז והוא הברזל הטוב במלאכה עד שיתוקן שהוא יותר חזק ויותר קשה מהברזל האחר... וזאת הדבקה יקרא חיסום להיות ע"פ כלי המחזיק בו ומונע אותו מהחזרה והבקיעה."^[Rambam, Commentary on the Mishnah, Kelim 13:4.]
The Rambam identifies chisum as acciaio (steel), specifically the Indian iron (parzla hindu'ah) used by blacksmiths to face soft iron tools. In his halakhic formulation in the Yad, the Rambam rules:
"המעצד והאזמל והמפסלת והמקדח שנפגמו, אף על פי שאינן ראוין למלאכתן כמו שהיו—עדיין הן טמאין... ניטל חיסומן, שהוא הברזל הקשה שחופין בו פני הכלי כדי שלא יתעקם—טהורין."^[Rambam, Mishneh Torah, Hilkhot Kelim 11:14.]
For the Rambam, shem kli is not a binary switch of "usable vs. unusable." It is an objective structural category. A tool that is merely nicked or damaged (shenifgemu) remains tamei because its core composition is intact, and it can still perform its primary task, albeit poorly.
However, when its chisum (steel facing) is lost, it undergoes a qualitative transformation. It is no longer just a "damaged tool"; it has reverted to soft iron. Even if one can still scrape wood with a soft iron adze, the loss of the chisum strips the object of its professional designation. It is no longer a tool of the charash (craftsman). It has lost its form, and with the loss of form, its tumah evaporates.
[ INTACT TOOL ]
(Steel + Soft Iron Core)
│
├─► Nicked / Damaged (Shenifgemu) ──► STILL TAMEI (Form Intact)
│
└─► Steel Edge Lost (Nital Chisumo) ─► TAHOR (Reverted to Soft Iron)
Furthermore, the Rambam adopts the reading: "וכולם שנחלקו לשנים – טמאים" (if they split in two, both remain tamei).^[Rambam, Mishneh Torah, Hilkhot Kelim 11:15.] He explains that if any of these tools (except the drill) split down their length, each half remains susceptible to impurity.
How can a split tool be tamei? The Rambam's conceptual model is that each split half can perform a minor version of the tool's original labor. A split adze can still scrape; a split chisel can still gouge. The ontological unity of the tool is indeed broken, but the functional utility of the metal fragment is preserved.
Rash mi-Shantz: Functional Minimalism and Textual Emendation
The Rash mi-Shantz presents a radically different reading of the split tools. He writes:
"וכולם שנחלקו לב' אין עושין מעין מלאכתן חוץ מן המקדח... נראה שגורס טהורים. וכך הגיה מהר"ם."^[Rash mi-Shantz, Kelim 13:4, s.v. "וכולם שנחלקו לשנים".]
The Rash rejects the Rambam’s reading of "טמאים" (unclean) for split tools. He argues that once these precision tools are split in two, they are completely incapable of performing me'ein melakhtan (anything resembling their original function). The only exception is the drill (makdeach), which, even if split, can still be spun to bore a hole in soft material.
For all other tools, the physical split destroys the shem kli entirely, rendering them tahor.
The Rash’s approach is rooted in functional minimalism. He does not look at the theoretical capacity of a metal fragment to scrape or scratch. He asks: Is this fragment still identified by the market as a tool of its class?
Once a chisel is split down the middle, it can no longer be struck by a mallet without shattering, and it cannot be held in the hand in a standard way. It is no longer a kli; it is a shever kli (a fragment of a vessel). And a shever kli of metal is only tamei if it can perform a task that is me'ein melakhto ha-rishonah (resembling its original work) in an efficient, recognizable manner.^[See Shabbat 112a for the general principles of shiyurei kelim.]
Tosafot Yom Tov: Spatial Mechanics of the Hasit and the Etymology of Megirah
The Tosafot Yom Tov provides a precise linguistic and mathematical analysis of the saw (megirah). He cites the Rash:
"מגירה... מליאה פגימות כשרוצים לחתוך קורה לשנים... ונקראת מגירה על שם שחותכת דרך גרירה."^[Tosafot Yom Tov, Kelim 13:4, s.v. "מגירה".]
The saw operates through gerirah (dragging). The Mishnah rules that if a saw’s teeth are missing "one in every two," it is tahor (clean). However, if there remains a continuous stretch of teeth equivalent to a hasit, it remains tamei.
The Tosafot Yom Tov dives into the exact dimension of this hasit. Citing the Rambam, he writes:
"שמלא הסיט ארבע אצבעות בגודל ונראה לרוב הקדמונים שרוחב הסיט מקצה הגודל אל קצה האצבע כאשר יפתחהו האדם בתכלית מה שאפשר..."^[Tosafot Yom Tov, Kelim 13:4, s.v. "חיסומן" (referencing Rambam's commentary on the hasit).]
The hasit is not a uniform measurement; it represents a functional spatial unit. In Shabbat 105b, the Gemara distinguishes between mlo hasit (the full span of the thumb and index finger) and rohav hasit (the width of the double span).
The Tosafot Yom Tov explains that a saw needs a continuous block of teeth of at least this size because a shorter span cannot be dragged back and forth across a piece of wood to produce a cut. If the teeth are scattered with gaps of "one in every two," the saw cannot glide; it catches on the wood and is functionally useless.
Here, the tumah is not determined by the total volume of metal or the total number of teeth, but by the spatial continuity required for the physical mechanics of sawing.
SCATTERED TEETH (Gaps of "one in every two"):
[Tooth] ── [Gap] ── [Tooth] ── [Gap] ── [Tooth] ===> TAHOR (Saw catches and cannot glide)
CONTINUOUS TEETH (Span of at least one "Hasit"):
[Tooth][Tooth][Tooth][Tooth][Tooth] ===> TAMEI (Sufficient span to saw wood)
Chazon Ish: Functional Dualism and the Metaphysics of Yichud
The Chazon Ish, in his commentary on Kelim, provides a deep conceptual framework for the needle (mahat) and the pack-needle (sakrin) in Mishnah Kelim 13:5.
The Mishnah states that a regular needle whose eye (nekavah) or point (chuda) is missing is tahor. However, if one adapts it (hitkinah) to be a stretching-pin (maltheach), it becomes tamei.
Conversely, a pack-needle whose eye is missing remains tamei even without physical adaptation, because "one writes with it."
The Chazon Ish raises a fundamental question: Why does the pack-needle remain tamei without physical adaptation, while the regular needle requires physical adaptation (hitkinah)?
He explains that there are two distinct paths for a damaged vessel to retain or regain its susceptibility to impurity:
- Objective Structural Capacity (Mizgiyah): If an object, by virtue of its size and weight, is naturally suited for an alternative, common task, it does not lose its status as a kli when its primary function is destroyed. The pack-needle is thick and heavy. When its eye is lost, it can immediately serve as a stylus (kotev) to score lines on leather or parchment. Because this alternative use is common and requires no physical alteration of the needle's body, the shem kli never left it. It simply shifted from "sewing tool" to "writing tool."
- Subjective Mental Designation (Yichud/Tikkun): A standard sewing needle is thin and delicate. When its eye is lost, it cannot be used as a stylus—it would bend or pierce the writer's fingers. Its only potential use is as a stretching-pin (maltheach) to hold fabrics together. But because a standard needle is not naturally associated with this task by default, the mere potential is insufficient. It requires hitkinah—either a physical bending of the head or a formal, subjective act of designation (yichud) that re-establishes it as a functional utensil in the eyes of the owner.^[Chazon Ish, Kelim, Siman 18, s.v. "מחט שנשבר קופה".]
Through this, the Chazon Ish establishes that tumah is governed by a dynamic tension between objective physical design and subjective human utility.
Friction
Kushya 1: The Paradox of the Pack-Needle (Sakrin) vs. the Regular Needle (Mahat)
We must confront a glaring contradiction in Mishnah Kelim 13:5. The Mishnah states:
"סַקְרִין שֶׁנִּטַּל קֻפָּהּ – טְמֵאָה, מִפְּנֵי שֶׁכּוֹתֵב בָּהּ. נִטַּל חֻדָּהּ – טְהוֹרָה."
If the pack-needle (sakrin) whose eye (nekavah/kupa) is missing remains tamei because "one writes with it," why is it tahor when its point (chuda) is missing?
If the writing is performed with the blunt end (the broken eye-end), the presence or absence of the point should be completely irrelevant to its writing capacity!
Conversely, if the writing is performed with the sharp point, then when the eye is missing, it should indeed be tamei. But then, why is a regular needle whose eye is missing tahor? A regular needle also has a point, and one could write or scratch with it just as easily!
┌───────────────────────────────┐
│ THE NEEDLE PARADOX │
└───────────────┬───────────────┘
│
Which end is used for writing?
│
┌───────────────┴───────────────┐
▼ ▼
[ THE BLUNT END ] [ THE SHARP POINT ]
│ │
If so, why is it TAHOR when If so, why is a regular needle
the point is missing? with a point TAHOR when its
(The blunt end is still intact!) eye is missing?
Terutz A: The Structural Dynamics of Writing (Rosh & Tiferet Yisrael)
The Rosh resolves this by analyzing how a pack-needle is held and used for writing.^[Rosh, Kelim 13:5, s.v. "סקרין שניטל קופה".]
Writing with a metal stylus on leather or clay requires significant downward pressure. If the pack-needle loses its point (chuda), the entire balance of the tool is ruined.
Furthermore, the "writing" mentioned here is not ink writing; it is the scratching of guidelines (sirtut) on parchment or leather.
To scratch a line, one needs to hold the needle at an angle. If the point is missing, the needle is shortened, and the hand cannot grip it securely without slipping.
Moreover, the Tiferet Yisrael explains that when a pack-needle is used to sew sacks, the point must be extremely sharp to pierce heavy canvas. If the point is lost, the tool is completely useless for sewing.
Could it still be used for writing? No, because the writing is done with the point itself, not the blunt end!
Why then is a regular needle whose eye is missing tahor despite having a point? Because a regular needle is too thin; if you apply the pressure required to score parchment, the needle will bend or snap.
Therefore, a regular needle cannot be used for writing, while a pack-needle can. Thus, for a pack-needle, the point is the critical component for both sewing and writing. When the point is lost, all utility is lost, and it becomes tahor.
Terutz B: The "Two-Handed" Grip (Chazon Ish)
The Chazon Ish offers a different resolution.^[Chazon Ish, Kelim, Siman 18, Ot 4.] He argues that the writing is indeed performed with the broken, blunt eye-end, which acts as a smooth stylus that does not tear the parchment.
Why then does the loss of the point render it tahor?
Because of how the scribe grips the tool. To write with precision, the scribe must rest the index finger near the tip of the tool. If the tip is a sharp point, the scribe cannot rest their finger there unless the point is embedded in a protective wooden handle or cap.
However, if the point is completely missing (meaning the needle was broken at its tip), the metal is jagged and cannot be capped or held comfortably.
Alternatively, the Chazon Ish suggests that the shem kli of a pack-needle is fundamentally unified. Even if a secondary function (writing) remains when the eye is lost, this secondary function is only recognized by halakha if the tool retains its general, professional appearance.
A pack-needle with a missing eye still looks like a professional stylus. But a pack-needle with a missing point looks like a useless piece of scrap metal. The loss of the point destroys its aesthetic and professional identity, rendering it tahor regardless of any theoretical secondary use.
Kushya 2: Rabbi Joshua’s Bafflement and the Axiom of Nullification
In Mishnah Kelim 13:7, we find a profound statement:
"וְעַל כֻּלָּם אָמַר רַבִּי יְהוֹשֻׁעַ: דָּבָר חִדְּשׁוּ סוֹפְרִים, וְאֵין לִי מַה אָשִׁיב."
What is this "new principle of law" (davar she-hidshu sofrim) that left Rabbi Joshua—one of the greatest tannaim—without an explanation?
The Mishnah is referring to a wooden pitch-fork (atza), winnowing-fan (rechat), rake (magrefah), or comb (masrek) that lost one of its wooden teeth, which was then replaced with a metal tooth. The Mishnah rules that this replacement renders the entire wooden vessel susceptible to tumah.
The friction here is conceptual: Why should a single metal tooth render a flat wooden vessel tamei?
According to Torah law, wooden vessels are only susceptible to impurity if they have a receptacle (beit kibul), as derived from the verse in Leviticus 11:32: "מִכָּל כְּלִי עֵץ" (of any vessel of wood). Flat wooden tools (peshutei klay etz) are completely tahor.
Metal vessels, however, are susceptible to impurity even if they are flat (peshutei klay matat), as derived from Numbers 31:22: "אַךְ אֶת הַזָּהָב וְאֶת הַכָּסֶף..." (only the gold and the silver).
If we have a wooden rake (which is flat and therefore tahor) and we insert one metal tooth, we should apply the standard rule of batel be-rov (nullification by the majority). The rake is 95% wood and only 5% metal! The metal tooth should be nullified by the wooden body, rendering the entire object tahor.
Why does the metal tooth "drag" the wooden vessel into susceptibility, violating the foundational axiom of rov?
[ THE RAKE COMPOSITION ]
┌──────────────────────┐
│ 95% Wood (Flat) │ ──► Normally TAHOR
│ 5% Metal (Tooth) │ ──► Normally TAMEI
└──────────────────────┘
│
Does the metal tooth dominate?
│
┌───────────────┴───────────────┐
▼ ▼
[ STANDARD ROV ] [ SCRIBAL NOVUM ]
Metal is nullified. Metal tooth drags wood.
Vessel remains TAHOR. Entire vessel is TAMEI.
Terutz A: The "Active Agent" Principle (Rashi & Chulin 124b)
The Gemara in Chulin 124b and Shabbat 60a addresses this directly. Rashi explains that the "scribal novum" is that the metal tooth is the ikar (the primary functional part) of the tool.^[Rashi, Chulin 124b, s.v. "דבר שחידשו סופרים".]
When a rake or comb loses a tooth, it cannot function. The replacement metal tooth is what allows the entire tool to gather leaves or comb hair.
Therefore, even though the metal tooth is physically the minority of the vessel, it is the functional majority. Halakha does not measure rov (majority) solely by physical volume or weight; it measures rov by functional significance.
Because the entire tool now relies on this metal tooth to perform its task, the metal tooth dominates the identity of the vessel. Rabbi Joshua was baffled because this principle—that a minor physical component can dictate the halakhic material classification of an entire vessel—runs counter to the standard rules of nullification (bittul) found elsewhere in the Torah.
Terutz B: The "Solfeggio" of Metal and Wood (Rash & Rambam)
The Rash and Rambam offer an alternative conceptualization. They explain that Rabbi Joshua’s astonishment was directed at the Rabbinic decree (gezerat sofrim) that treated this composite tool as a metal vessel.
Under Torah law, if a metal tooth is inserted into a wooden rake, the metal tooth is considered tamei on its own, but the wooden body remains tahor. The Scribes, however, decreed that the entire tool is tamei as a single, unified entity.
Why did they do this? To prevent a loophole. If a person used this rake and it touched a source of tumah, they might assume the handle is clean and continue to use it, forgetting that the metal tooth is carrying impurity.
Because the metal tooth is permanently integrated into the wood, the Scribes treated the wood as an extension of the metal (yad le-felez).
Rabbi Joshua had "no explanation to offer" because this decree treats wood (which is flat and should be immune) as metal, creating a hybrid category of impurity that has no direct precedent in Biblical law.
Intertext
Shabbat 60a: The Shabbat-Kelim Interface
The mechanics of our Mishnah in Kelim directly impact the laws of carrying on Shabbat. In Shabbat 60a, the Gemara discusses whether a woman may walk out into the public domain wearing a "comb" (masrek) or a "stretching-pin" (maltheach).
The Gemara asks: Is this item considered an ornament (takhshit), in which case she may carry it, or is it a utensil (kli), in which case carrying it violates the Rabbinic prohibition against carrying items that might be slipped off and shown to a friend?
[ COMB / PIN ]
│
Is it an ornament or a utensil?
│
┌───────────────┴───────────────┐
▼ ▼
[ ORNAMENT ] [ UTENSIL ]
(Takhshit) (Kli)
│ │
May wear in public. Muktzeh / Cannot wear.
The Gemara connects this directly to our Mishnah in Kelim:
"תנן: מַחַט שֶׁנִּטַּל קֻפָּהּ אוֹ חֻדָּהּ – טְהוֹרָה. אִם הִתְקִינָהּ לְמַלְתֵּחַ – טְמֵאָה. שמע מינה מלתיח כלי הוא!"^[Shabbat 60a.]
The Gemara proves that a stretching-pin (maltheach) is classified as a kli (utensil) because our Mishnah rules that if one adapts a broken needle to be a stretching-pin, it becomes tamei.
If it were merely an ornament, it would not be susceptible to the impurity of vessels. Because it is a kli, a woman is rabbinically forbidden to wear it in the public domain on Shabbat, as she might use it to pin her garments and then remove it, leading to carrying in public.
This intertextual link demonstrates that the ontological classifications of Kelim are not limited to the laws of ritual purity. They define the very nature of "utensils" for the laws of Shabbat, including the prohibitions of carrying and muktzeh.
Shulchan Aruch, Yoreh Deah 120: Composite Vessels and Tevilat Kelim
The structural dynamics of "wood serving metal" vs. "metal serving wood" outlined in Mishnah Kelim 13:6 form the bedrock of the laws of Tevilat Kelim (immersion of newly acquired vessels from a non-Jew).
The Mishnah states:
"עֵץ הַמְשַׁמֵּשׁ אֶת הַמַּתָּכֶת – טָמֵא. וּמַתָּכֶת הַמְשַׁמֶּשֶׁת אֶת הָעֵץ – טָהוֹר."
The Shulchan Aruch codifies this principle regarding glass and metal vessels that are combined with wood or plastic:
"כלי עץ שחופו אותם באבר [בדיל] או שהעמידו להם ידות של מתכת... אם עיקר הכלי הוא של עץ והמתכת הוא רק טפל לו—אינו טעון טבילה. אבל אם המתכת הוא עיקר הכלי—טעון טבילה."^[Shulchan Aruch, Yoreh Deah 120:3.]
The Shach, commenting on this ruling, traces it directly back to our sugya in Kelim:
"עיין במסכת כלים פרק י"ג... דכל שהעץ משמש את המתכת, כגון בית יד של ברזל שהוא של עץ, הולכים אחר הברזל שהוא עיקר המלאכה."^[Shach, Yoreh Deah 120:3, s.v. "אם עיקר הכלי".]
The Shach establishes a critical halakhic heuristic: The Essence Follows the Function (Azlinan batar tashmish).
If a wooden tray has metal handles, the handles are auxiliary to the wooden tray; the wood serves the food, and the metal serves the wood. The metal is tafel (secondary), and the vessel does not require tevilah.
However, if a metal pot has wooden handles, the wood serves the metal (by protecting the cook's hands from the heat). The metal is the ikar (primary), and the entire vessel requires tevilah with a blessing.
WOOD SERVES METAL (e.g., Metal pot with wooden handles):
[Metal Pot] <─── [Wooden Handles] ===> Metal is primary. Requires Tevilah with a Bracha.
METAL SERVES WOOD (e.g., Wooden tray with metal handles):
[Wooden Tray] <─── [Metal Handles] ===> Wood is primary. Exempt from Tevilah.
Psak/Practice
Modern Modular Tooling: The Case of the Utility Knife
How do these ancient principles of metallurgy and structural identity apply to modern consumer goods? Let us analyze a contemporary utility knife (e.g., a "Box Cutter" or "Stanley Knife") with a plastic body and a retractable, snap-off steel blade.
┌───────────────────────────────┐
│ MODERN UTILITY KNIFE │
└───────────────┬───────────────┘
│
Is the plastic body a "Kli"?
│
┌───────────────┴───────────────┐
▼ ▼
[ BLADE INSTALLED ] [ BLADE REMOVED ]
│ │
Vessel is TAMEI. Is it still TAMEI?
│
┌───────┴───────┐
▼ ▼
[ YES ] [ NO ]
(Rambam/Kelim) (Modern Poskim)
Applying the principles of Mishnah Kelim 13:4:
- The Blade as Chisum: The steel blade is the chisum of the utility knife. Without the blade, the plastic handle cannot cut anything.
- The Status of the Handle: When the blade is completely removed and discarded, does the plastic handle remain a kli?
- According to the Rambam's ruling on the plane (rokani), "הַדְּפוּס שֶׁל מַחְלֵקָה בִּפְנֵי עַצְמוֹ – טָהוֹר" (the wooden block of a plane by itself is tahor).^[Mishnah Kelim 13:4.] The block of the plane only holds the iron blade; when the blade is removed, the block has no independent utility and is tahor.
- Therefore, a plastic utility knife handle without its blade is completely tahor.
Halakhic Decisions on Composite Materials and Electrical Appliances
In modern responsa, the status of composite materials (such as metal heating elements embedded in plastic electric kettles) is heavily debated.
The Minchat Yitzchak addresses whether a plastic electric kettle requires tevilah because of its metal heating element:
"בנדון קומקום חשמלי של פלסטיק שיש בו גוף חימום של מתכת... יש לדון בזה משום 'עץ המשמש את המתכת' או 'מתכת המשמשת את העץ'..."^[Minchat Yitzchak, Vol. 2, Siman 72.]
The Minchat Yitzchak rules that the plastic body of the kettle serves to hold the water, while the metal heating element serves to heat it.
Because the primary function of a kettle is to boil water—which cannot happen without the electrical heating element—the plastic is considered to be "serving the metal" (משמש את המתכת).
Consequently, the entire appliance is classified under the halakhic category of a metal vessel and requires tevilah (though some poskim rule it should be done without a blessing due to the risk of ruining the electrical circuitry).
Takeaway
Susceptibility to impurity is not dictated by mere physical volume, but by the structural integrity of a vessel's primary function; when that function is lost, only a formal act of human designation can restore its halakhic identity.
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