US2346955A - Thermistor - Google Patents

Description

Patented Apr. 18, 1944 THERMISTOR Henry Wolfson and Stanley Garden Shepard, London, England, assignors to International Standard Electric Corporation, New York, N. Y.

No Drawing. Application July 1, 1942, Serial No. 449,352. In Great Britain August 22, 1941 3 Claims.

The present invention relates to a process of manufacturing heater or the like resistance coils such as, for example, the heater coils of indirectly heated resistances or the resistance of a resistance thermometer. More particularly it relates to such resistances which have a pronounced negative or positive temperature coefficient of resistance. Such devices have been, for convenience described as Thermistors.

In one form of construction of indirectly heated thermistors, in which the resistance element is in the form of a small bead having attached conductive lead-wires, a prewound heater coil of small dimensions surrounds the bead, and is held in the desired position relative to the bead, and is insulated therefrom by means such as an insulating cement. In cases where, for instance, the resistance element is extremely small, say 0.03" dia., exact centering of the bead in the heater coil is difficult, particularly as the heater coil is many times longer than the bead. In such cases the heat of the coil is not concentrated on the resistance element, and the manufacture of thermistors with reproducible characteristics is difficult.

It is the object of this invention to produce heater coils in which the turns lie closely together, though still separate and insulated one from the other, of much smaller dimensions than the heater coils hitherto used and of readily reproducible heating characteristics.

According to the invention the process of manufacturing a resistance coil of the type specified comprises applying to the coil a material, which after fusion and subsequent solidification has insulating properties, and applying heat to fuse the material so that the surface tension of the fused material pulls the coil turns close together leaving a thin layer of the material between successive turns.

In one practical method of carrying the invention into effect a heater coil which consists of a fine gauge resistance wire such as a nickelchromium alloy wire known under the registered trade-mark Nichrome or a similar resistance wire, or tungsten, for example, is dipped into an aqueous solution, which may be saturated, of sodium biborate or other metal borate solution or suspension (e. g. lithium, potassium, lead, etc), and is then transferred to a furnace or oven at a temperature above the fusion point of the borate but below the melting point of the wire. When the borate fuses, the turns of the coil are pulled together by surface tension but are insulated from one another and prevented from touching by a thin film of the borate.

The process can also be carried out by using any fusible glass or enamel which will exert sufficient surface tension to pull the coil turns together, and which will insulate the turns one from the other.

A rigid glazed heater of tube like form is produced in this way, into which it is easy to insert the thermistor element with its attached lead wires, and in which accurate centering by means such as an insulating cement is carried out easily and in a readily reproducible manner.

As an alternative method of carrying the invention into effect, the coil is first dipped into water and then in the finely powdered borate or other fusible material or a mixture of fusible materials, and then heated in a furnace or oven.

Instead of heating the coil and fusible material in a furnace or oven the fusion of the material and the shrinkage of the coil may be performed in a flame.

Whilst typical examples employed in carrying out the process according to this invention have been given other combinations will be apparent to those skilled in the art. It will be observed that the materials must be so chosen that the flexibility of the coil is sufficiently great for the surface tension of the fused material to pull the turns together.

The advantages of the process of manufacture of heater coils according to this invention are: heater coils of the same resistance as heretofore can be made of much smaller length; the size of the assembly when used with resistors, for instance, is reduced; the heater is covered with an insulating layer and is in the form of a rigid hollow tube, non-uniformities in coil lentgh consequent upon cutting a large length into smaller lengths of equa1 resistance are removed, since all coils having the same amount of wire shrink to the same size.

What is claimed is:

1. The herein described method of making preformed heater coil tubes for indirectly heated resistors which consists in applying to a heater coilhaving the requisite degree of flexibility a material which after fusion and subsequent solidification has insulating properties and applying heat to fuse the material whereby the surface tension of the fused material pulls the coil turns closely together to a horned stop position with a thin layer of the material between the successive turns.

2. The process of manufacturing a preformed layer of insulating material between the sucheater coil tube for example for an indirectly cessive turns.

heated resistance such as a, thermistor which 3. The process according to claim 1 wherein comprises covering a spiral coil of tungsten wire the coil is composed of a nichrome wire of a fine of fine guage with a metal borate and applying 8 guage and the said material consists of metal heat to fuse the borate so that the surface tenborate.

sion of the material pulls the turns of the coil HElIRY WOLFSON.

together to a homed stop position leaving a thin STANLEY GARDEN SHEPARD.