US2349614A - Electric resistance element, more particularly of resistance thermometers - Google Patents

Description

i 1944- A. c. DENISOFF. ETAL 2,349,614

ELECTRIC RESISTANCE ELEMENT, MORE PARTICULARLY.

' 0F RESISTANCE THERHOMETERS Filed Dec. 28, 1942 Patented May 23, 1944 ELECTRIC RESISTANCE ELEMENT, MORE PARTICULARLY .OF RESISTANCE THER- Alexander Constantine Denlsoi! and Bernard Pat- Scanian,

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London, England. usignors to Simmonds Aerocessories, Limited, London, England Application December 28, 1942, Serial m. 410,302 In Great Britain December 10, 1941 2 Claims. (o 201-63) This invention relates to electric resistance elements, and more particularly to the resistance elements of resistance thermometers.

It is known that small amounts of impurities, of the order of one per cent or less, in a metal, for example, nickel or a nickel alloy, have a definite eifect on the temperature-resistance characteristic of the metal. In practice, unless extreme precautions are taken in the manufacture of the metal, every new batch has a somewhat difierent temperature coefilcient and it is therefore a matter of great diillculty to manufacture a resistance element that shall have a consistently "uniform temperature resistance curve.

An object of the present invention is to provide a simple method of manufacturing resistance elements, more particularly for resistance thermometers, which shall have a consistently uniform temperature-resistance characteristic despite slight changes in the materials employed.

To this end the effect 01' small amounts of impurities on the temperature coeificient is utilized by forming the improved resistance element of two or more pieces of resistance material which are of essentially the same composition but difier as regards the amount of impurity present and hence differ as regards their temperature coefilcients. Such pieces are connected in series and the resistance of each piece adjusted such that the element as a whole has the desired temperature-resistance characteristic. Thus a change in the degree of impurity present in either or both of the pieces of resistance material merely involves a modification in the resistance, in practice the length, of the pieces and the periormance oi theelement may thereby be retained unimpaired at the desired value.

The adjustment of the resistance of the pieces oi resistance material to eflect this result will now be explained by the following theoretical analysis and example.

The variation of electrical resistance R with temperature t may be accurately represented for almost all metals by means of a parabolic function where a. and p are constants and are known respectively as the first and second order temperature coefllcients of resistance the metal.

For an element consisting of two wires having different 'coefiicients connected in series, the function R:=l(t) may be written where R1, (:1, and Bi, and R2, a2 and )3: are the resistance at 0 C., and first and second order coeificients of the two wires respectively.

The mean first order coefiicient am for the eleelement is clearly given by It may be remarked here that if the two wires were to have been connected in parallel the relation between resistance and temperature could no longer have been expressed by a parabolic function and it would not be possible to define a mean first or second order, coefilcient as above.

Over temperature ranges of about 100 C. it has been iound that for some metals (e. g. nickel) slight variations in the impurities do not afiect the value of the second order coefllcient. Hence for elements working over this temperature range the problem of producing a desired temperatureresistance characteristic is reduced to an adjustment oi the first order coefilcient.

For example, suppose that it is desired to produce .a resistance thermometer having a mean first order coefilcient of x10 and a resistance of ohms at 0 C. and that wires-having first order coemcients oi 53.0X 10* and 45.0x l0- are available. Let the element resistance at 0 C. be Ra then R0=Ri+RI and substituting for R: in the expression given above for the mean first order coefilcient, we may write i i-' and similarly Hence by connecting in series the two wires each of such length that the first, having the coefilcient of 53.0x10- is of 62.5 ohms resistance and the second is oi 37.5 ohms resistance, a resistance so element having the desired properties may be obtained.

It will be clear from the above that similar results may be achieved by connecting three or more diilerent wires in series for as has been shown above the composite or mean first and second order coemcients, am and flm, always remin as first and second order coefilcients however many difierent wires are used, and hence the resulting temperature-resistance characteristic still remains parabolic.

It will also be clear that ii it is desired to correct for variations in the second order coeiiicient as may occur when the element is to be used to cover a wide range of temperature similar methods may be employed.-

It will be appreciated that by a careful adjustment of the resistance oi the different wires forming the element, in accordance with this invention, it is possible to make a very fine and simple adjustment of the coefllcient of the element. Moreover the coefiicient of the manufactured element may be readily maintained at a fixed value despite variations in the quality of different batches of metal for it is only necessary to measure the coefiicients of a new batch once and for all when the necessary modifications in resistance and hence the lengths of the wires to be used may be determined.

One method of mounting a resistance element constructed in accordance with this invention is illustrated by the accompanying drawing which shows a resistance thermometer in elevation, part thereof being cut away to show the internal arrangement. The resistance element is in the form of a helix I laid in grooves 2 formed in an insulating support 3. The grooves are in the form' of a two start thread merging at the upper end a with longitudinally arranged grooves leading to the head of the support. The support is protected by a sheath I which is provided at its i upper end with an enlarged head 5 within which is accommodated sockets, connected with the wire helix, for the reception of a plug or other means of connection with indicating apparatus.

We claim: I l. The method of making a resistance element having a desired temperature-resistance characteristic which consists in forming the element oi two series-connected pieces of resistance material which are essentially oi the same composition but differ somewhat as regards their first order temperature coefllcients of resistance and in choosing the resistance of the said two pieces to satisfy the relation a, l l+ 2 2 m RI+R2 where am, aial'ld as are respectively the desired temperature coeflicient oi the element, the coefiicient of the said first piece and the coefllcient of the said second piece and R1 and R2 are the resistarices of the said first and saidvsecond pieces respectively.

2. A resistance element having a desired first order temperature coeiiicient of resistance comprising a first and a second piece of resistance material, connected together in series, the said first and second pieces being of substantially the same composition but differing somewhat as regards their first order temperature cofilcients oi resistance, the said first piece having a coefil'cient slightly higher than the said desired coefilcient of the element and a resistance equal to (I -"d2 l"' 2 and the said second piece having a coemcient slightly lower than the said desired coefiicient and a resistance equal to and the coefilcient of the said second piece and R0 is the resistance of the element as a whole.

ALEXANDER CONSTAN'I'INE DENISOFF. BERNARD PATRICK SCANLAN.

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