274 Mr. A. Campbell on the Measurement of very 



In an exactly similar way very small alternating currents 

 may be measured. 



In order to avoid errors due to stray magnetic fields the 

 transformer may be wound with double coils placed side by 

 side with their axes parallel and reversed in the well-known 

 manner. 



It might also be well to have the position of one or more 

 turns of the primary or secondary adjustable with a scale to 

 show the proper position for each particular frequency. 



For the purposes of calibrating at any frequency a second 

 primary of a different number of turns from the first may be 

 wound on the transformer, and the relationship of the two 

 primaries may be determined once for all. 



For example, if the first primary is for 1000 amperes the 

 second may have 100 times the number of turns and be suit- 

 able for 10 amperes. The correction for any particular 

 frequency or wave-form may then be found experimentally 

 by using the second primary only. 



I may mention that with very simple instruments I have 

 used an air-core transformer to measure currents between 

 1000 and 2000 amperes. 



[Added June 12th, 1896.] 

 In order to get a larger increase of secondary voltage a 



well-known method is to connect up a transformer with the 

 primary and secondary in series as in fig. 1. By a somewhat 



