132 



ELECTRICAL MEASUREMENTS 



and at the Bureau of Standards, Washington. Fig. 59 shows 

 a group of these coils. 



For standards having a resistance of J^o nm an d greater, the 

 resistance material is used in the form of wire; below J-f ohm, 

 strips are employed. 



The coils are kept cool by immersion in oil. Great care must 

 be taken that there are no acids or sulphur in the oil and that it 



FIG. 59. Standard resistances. 



is kept free from water. Otherwise the resistance material may 

 be attacked and the accuracy of the standards impaired. The 

 oil should be renewed from time to time. 



As a result of careful experiments at the Bureau of Standards 

 it has been found that coils constructed like that shown at A in 

 Fig. 59 are subject to slight variations due to the absorption of 

 moisture by the shellac used in insulating the windings. This 



