A POCKET GALVANOMETER. 



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Fig. 2. 



fixei central axis perpendiciilai' to its plane and parallel to tlie 

 magnets, as shown in 

 Fig. 2. What we want 

 to measure is the couple 

 about this axis. We 

 may accomplish this by 

 means of tw^o springs, 

 one attached to the 

 frame, the other to an 

 index, and both springs 

 initially stretched very 

 much as in an ordi- 

 nary aneroid barometer. 

 The motion of the index 

 is magnified by means of a multiplying lever introduced between the 

 index and its spring, as vshown in Fig. 4 below. As we cannot use 

 steel springs, an alloy of gold and brass, which is used in making 

 springs for pince-nez eyeglasses, answ^ers here very well. Ordinary 

 brass springs however are easier to make, although they are not 

 quite so lasting. 



The whole arrangement is then put in a case with a jaw-like 

 vacant space hollowed out for receiving a conducting wire. The axis 

 for the index projects above a graduated circular plate, and the index 

 is fixed at right angles to the jaw, in such a way that when part of a 

 circuit is slipped in within the jaw, the index will be moved toward 

 that side to which the current is passing ; in other words we may 

 imagine the tip of the index to be carried by the current. The value 

 of the indications depends upon the strengths of both the magnets 

 and the springs, and has to be obtained by comparison with some 

 standard galvanometer ; and this calibration must be repeated occa- 



