foote: an optical ammeter 



79 



brings the lamp to incandescence. By adjusting the rheostat G 

 the brightness of the lamp C is matched against the image of the 

 background E illuminated by F. The current required for the 

 match is read potentiometrically by use of the resistance stand- 

 ard K. Hence the value of the alternating current is determined 

 by an auxiliary measurement of direct current. The relation 

 between the currents thi'ough F and C is obtained in the pri- 

 mary standardization of the instrument by using a direct cur- 

 rent thi'ough C and measuring this potentiometrically. For the 

 highest precision a commutating switch may be used with C, 



To electrical Cu-cuit 



D 



AA 



I — VW\AA/\AAWW\AA 



m:^ 



Potentiometer 



standard 

 Resistance 



Fig. 1. An optical ammeter. 



SO that the standardization with direct current may be performed 

 immediately following the observations with alternating current. 

 The range of currents which can be measured by any one system 

 of lamps exceeds 1:2, and lamps having filaments of varying 

 sizes may be used for a large range in value of currents. Thus 

 one lamp may be employed for alternating currents between 0.25 

 and 0.50 ampere, another lamp between 0.5 and 1 ampere, etc. 

 Or with the same lamp, the range may be extended by the use of 

 shunts, although this latter method is probably objectionable 

 for very high frequency. If the induction effect becomes seri- 

 ous for the horseshoe filament, a lamp having a straight wire 

 filament can be used. 



^^ hen a suitable magnification system is employed, the diffus- 

 ing screen E is unnecessary and a magnified image of F may be 



