232 MEASUREMENT OF CURRENTS. 



frame. Whatever be the winding and the position of the needle, the 

 actions of the two wires are equal. As the winding of the strand is 

 not very regular, it is usual to wind the two wires simultaneously and 

 parallel to each other on the frame. The system is then equivalent 

 to that of two identical coils, one of which has been displaced 

 parallel to itself by a quantity equal to the diameter of the wire ; it 

 follows from this that the actions of the two coils become unequal 

 when the centre of the needle is not in the plane of symmetry. This 

 inconvenience might, it is true, be remedied by taking care to invert 

 the order of the wires at each fresh layer that is to say, to put on 

 the right that which was previously on the left in the preceding layer ; 

 the adjustment of the needle need not then be so exact 



It is, moreover, unnecessary that the coils shall coincide. 

 Thomson's astatic galvanometer, consisting of two coils, with a 

 superposed double frame (849), may be used as a differential 

 galvanometer ; the currents must separately traverse the two coils 

 in such a direction that their actions on the corresponding needles 

 are in opposite directions. If M and M' are the magnetic moments 

 of the needles, H and H' the corresponding fields produced by the 

 combined action of the earth and of a correcting magnet, we have 



GMI - G'MT = (MH - M'H') tan 8. I 



Besides equalising the resistance, we should realise the condition 

 GM = G' M'; but a more symmetrical arrangement is obtained when 

 the two frames of each coil are connected. In order to facilitate the 

 adjustment, one of the systems has a less resistance than the other ; 

 it is then completed by a small additional coil, the action of which 

 coincides with that of the principal coil, and which can be approached 

 to or removed from the corresponding needle. Equality in the resist- 

 ances being thus obtained, a position is found by trial for this 

 auxiliary coil, so that the same current, traversing the two systems 

 in opposite directions, produces no deflection ; the galvanometric 

 constants are then equal. 



854. The differential galvanometer is most frequently employed 

 in establishing the identity of two currents ; it might, in case 

 of need, measure their difference. The arrangement adopted by 

 Mr. Fleming Jenkin* enables us to measure the ratio of the two 

 currents. 



* JENKIN. Report of the Committee of the British Association. Dundee, 1867. 



