2 Mr. A. Campbell on the Magnetic Fluxes in 



In some cases <I>, the total flux, was measured, and for several 

 of the meters determinations of the power lost in their various 

 parts were also made. 



Methods of measuring B. 



Method I. Except when the fluxes were alternating, the 

 method used was the well-known way with a ballistic galvano- 

 meter. A search-coil in circuit with the galvanometer either 

 had the flux suddenly reversed in it, or was pulled quickly 

 out of the field. For many of the experiments small and 

 very thin search-coils had to be used. For instance, in one 

 of the meters, the available air-gap ayrs only 1 millim. across. 

 The bobbins of these small coils were made by cementing 

 together one or more round microscope cover-glasses between 

 larger strips of mica for the ends ; they were wound with 

 from 10 to 200 turns of silk-covered copper wire of 0"075 

 millim. diameter. Much thicker wire was used when it was 

 desirable to have the resistance of the galvanometer-circuit 

 low. The smoothness of the mica cheeks allowed the coils to 

 be withdrawn from position with the necessary quickness. 



Fig. 1. 



The galvanometer was calibrated from time to time by means 

 of a standard pair of coils whose mutual inductance was 

 accurately known. As the measurement of small fluxes by 

 the ballistic method presents no difficulty, and requires only 

 ordinary instruments, further description is needless. 



Method II. When, however, the small flux is an alter- 

 nating one, the voltage set up in the search-coil is more 

 difficult to measure; accordingly two special methods were 

 here used. In the more accurate of these two methods 

 the search-coil Q (fig. 1), through which the flux was made 

 to pass, instead of being in circuit with a ballistic galvano- 

 meter, was joined directly to a resistance-coil R, laid along 

 one set of junctions of a minute^thermopile Th, which last 

 was connected w T ith a sensitive galvanometer G. The re- 

 sistance-ware was of manganin (silk-covered) and was placed 

 along the junctions so as to be non-inductive, and to avoid 



