Chap, x.] GALVANOMETER SHUNT. 103 



when the magnetism of the earth and that of the con- 

 trolling magnet are on the border land of neutralisa- 

 tion, the needles will be found to be unstable ; i.e. 

 the slightest movement of the magnet to one side will 

 cause the spot of light to dash across to that side, the 

 slightest movement to the other side will send it over 



o 



to that other side, and it will be impossible to bring it 

 by the influence of the magnet to the zero point. 

 This is because the influence of the earth has been 

 more than neutralised, and the needles consequently 

 come under the influence of the magnet, and tend to 

 turn right round to set their opposite pole under that 

 of the magnet. The position of greatest sensibility 

 will now be found by very carefully moving the 

 magnet up its support, by scarcely more than a hair's 

 breadth at a time, after. each movement giving the 

 fine adjustment the smallest possible turn, till the 

 instability disappears, when, by a very slight turning of 

 the screw, the beam of light can be brought to zero. 

 In this position the passage of an extremely feeble 

 current round the coils will cause a deflection of the 

 needles, and that deflection will take place slowly, so 

 that the spot of light will come to rest at the point of 

 maximum deflection after only one or two oscillations 

 on each side of it. 



A shunt is usually provided with each instrument, 

 with which one may regulate, within limits, the amount 

 of current sent through the galvanometer. This is seen 

 in Pig. 55. It has a series of brass plates separated 

 from one another, but, like those of the rheocord, 

 capable of being connected by brass plugs. When all 

 the plugs are out the plates are connected by varying 

 lengths of wire, so that a current forced to traverse 

 these wires encounters a certain amount of resistance. 

 The shunt has two binding screws. Two electrodes 

 are led from the apparatus producing the current, one 

 to each binding screw, and then from each binding 



