RESISTANCE DEVICES 129 



After it is wound, the coil is shellaced and then baked for 

 10 hours or more at 140C.; this frees the entire coil of moisture 

 and alcohol and at the same time anneals the wire. After baking, 

 the coil should be given a protective coating of paraffin. 



The resistance wires are hard-soldered to copper terminals 

 which in turn are soft-soldered to the working terminals. 



The prime requisite of the resistance material used for winding 

 coils is permanence. In addition, its temperature coefficient 

 should be small, its thermal e.m.f. when opposed to copper, low, 

 and to obtain compactness, its resistivity should be high. Alloys 

 rather than the pure metals are used for they have smaller tem- 

 perature coefficients and higher resistivities. To settle the all- 

 important question of permanence prolonged investigation is 

 of course necessary. Up to the present time, the alloy which 

 has most commended itself is that known as manganin. 2 

 Other alloys are used for certain kinds of work but manganin 

 has been under critical examination longer than the others and 

 its properties are more definitely known. 



Edward Weston discovered in 1889, that alloys of copper and 

 nickel containing some manganese have very small temperature 

 coefficients and high resistivities. Investigation has shown that 

 the particular alloy known as manganin is, when properly 

 employed, sufficiently permanent for resistance coils and resist- 

 ance standards. 



The composition of manganin is given as 84 per cent, copper, 

 12 per cent, manganese, and 4 per cent, nickel. Its resistivity 

 at 20C. is about 44.5 microhms (cm.) and its thermo-electro- 

 motive force when opposed to copper is only 0.000002 volt per 

 degree C. To insure permanence this material must be pro- 

 tected by a well-dried coating of shellac. 



The curious effect of a rise of temperature on a manganin 

 resistance coil is shown in Fig. 58. The point at which the tem- 

 perature coefficient changes sign varies with different samples of 

 wire. 



It will be noted that the temperature coefficient is very small, 

 the average value between 15C. and 20C. being only 0.0005 

 per cent. For engineering work, consequently, temperature 

 corrections may be neglected. In work of the highest precision 

 (a few parts in 100,000) temperature corrections must be made, 



