RESISTANCE DEVICES 131 



quently used in the better class of bridges and resistance boxes 

 for general use. 



No definite statement can be made as to the safe carrying, or 

 watt capacity, of the resistance coils used in boxes for general 

 laboratory work, for it depends on the construction of the coils 

 and of the box in which they are mounted and on the accuracy 

 of the initial adjustment of the coils. If the coils are of good 

 construction and wound on wooden bobbins 0.5 watt per coil may 

 be allowed. If metal spools in metallic connection with the con- 

 necting blocks on the top of the box are used, 3 watts per coil is 

 a safe allowance. These figures are for coils enclosed in wooden 

 boxes as is ordinarily the case. If many coils in the same box 

 are simultaneously used lower figures must be employed. 



By immersion in oil, which is well stirred, the watt capacity 

 of a coil on a metal bobbin is increased to about 7 watts. 



Standard Resistances 1 . Standard resistances are used for 

 two purposes: 



1. As standards of reference with which other resistances are 

 compared. 



2. As current carrying standards for use in potentiometer 

 methods. 



With the first class of coils, permanence is all important. The 

 watt capacity is not so important provided it is great enough to 

 allow comparisons to be made with the desired precision. 



Experience has shown that lack of permanence in the finished 

 coil may be due to corrosion, to stresses in the coil owing to the 

 fact that the wire is wound on a small spool, to stresses due to 

 the absorption of moisture by the insulating materials, and to the 

 use of soft solder at the terminals which may crack and alter the 

 effective length of the wire. These factors are now generally 

 recognized and the coils prepared accordingly. 



For coils of the second class, a high carrying capacity is abso- 

 lutely necessary and one can tolerate a less degree of permanence 

 provided comparisons are made from time to time with carefully 

 preserved resistance standards; as a matter of convenience per- 

 manence is highly desirable. 



The designs which are commonly employed for resistance stan- 

 dards are those developed at the Reichsanstalt, Charlottenburg, 



