Chap, x.] THOMSON'S GALVANOMETER. 101 



desired direction. The magnet may be brought near to, 

 or moved from, the needles by sliding down and up 

 the rod. Four binding screws have attached to them 

 the four ends of the wire of the coils. 



To use the instrument a lamp-and-scale arrange- 

 ment is employed (Fig. 54). 



The lamp and scale are placed facing the galvano- 

 meter at a distance of from two to three feet. A slit 

 below the scale permits a narrow beam of light to pass, 

 which is thrown on the mirror 

 of the upper needles, and from 

 it reflected on to the scale. The- 

 large magnet, which may be 

 turned by hand or by the fine 

 adjustment screw attached to 

 the cover, aids in bringing the 



7 O O 



beam to the zero point. AcrOSS Fig. 54. Lamp and Scale for 

 the Slit is Stretched a wire, Galvanometer. 



and the image of this ought to be focussed on the 

 scale. The current may be sent round both coils by 

 connecting the two middle binding screws, and then 

 joining the electrodes with the outer screws ; or the 

 instrument may be used to compare two currents, in 

 which case the electrodes for one current are to be 

 connected with the screws of one side, those for the 

 second current with the screws of the outer side. The 

 currents must be sent round both coils in the same 

 direction, so that the current passing round the 

 upper coil tends to deflect the needle in one 

 direction, and the current passing round the lower 

 coil tends to deflect the needle in the opposite 

 direction. If both are equal the spot of light will be 

 stationary on the scale ; if one is stronger than the 

 other the spot of light will travel over the scale, and 

 indicate the excess. By a preliminary experiment the 

 direction of deflection by each current can be deter- 

 mined separately, and thus it is easy to learn which is 



