A POCKET GALYANOMETEE. 



297 



diagram below. There are four, one around eacli magnet. They are 

 necessarily made of small height, in fact just enough to allow the 



wire magnets to move 

 ^'^' ^^' ^^^^^<^ freely inside and yet 



to leave a good space for 

 the jaw. For conveni- 

 ence of reference call 

 these coils A,B,C, D, 

 as indicated in the 

 figure. From the sym- 

 metry of configuration 

 we may take any one of the eight poles and consider the action of 

 four coils upon it. Take one of the poles inside A and call it P^ for 

 the sake of definiteness. The variations in the actions of C and 

 1) upon P^ due to possible motions of the same may be regarded as 

 the differentials of the actions of electromagnetic strips placed along 

 the edges of C and D. These we may safely neglect in comparison 

 with the variations in the actions of A and B. Thus we have only 

 to consider the effect of the four faces of ^ and B. But since each 

 coil extends a good way over the poles of the magnet, we may regard 

 these coils as drawn out indefinitely in the direction of the magnets' 

 axes without committing any sensible evvov as was shown in the last 

 section. Hence, calling the distances of the faces of A and B from 

 Pa, «i; «2? «35 «4- ^^ in the diagram, we have 



where «4 = «1 + % + «j from symmetry of arrangement. But from 

 the curves given above we see that when a/h is more than 4, the 

 values of-^^^-^ become insensible. Therefore by simply extending 

 the ends of the coils so as to have the least value of a/b more than 



