BY DR. MICHAEL FOSTER. 377 



adjusting magnet. On continuing to lower the magnet, the 

 point of neutralization is soon passed, and thc'ii tin- influence 

 of the adjusting magnet on the needle becomes stronger than 

 that ef the earth. The needle, consequently, which previously 

 had its north pole under the north pole of the magnet, would, 

 if free to turn, swing half round in the attempt to bring its 

 south pole under the north pole of the magnet; and indeed 

 does swing round as far as the arrangements of the apparatus 

 will allow, the spot of light rapidly travelling quite beyond 

 the limits of the scale. When this had been found to occur, 

 the magnet must be raised again np to or rather above the 

 point of neutralization. The oscillations of the needle will 

 now be at their minimum of rapidity, and the needle will be 

 at its maximum of sensitiveness. Bring the spot of light ex- 

 actly to zero. The magnet may be at first moved with the 

 hand, but this will be found to be too coarse a method. Finer 

 adjustment is gained by turning the milled head/. 



The wires conveying the current through the galvanometer 

 are to be attached to the outer binding screws b b. 



To determine which direction of current is indicated by the 

 movement of the spot of light, try the effect of a very feeble 

 cell, upon the galvanometer. But be careful not to use the 

 whole of the current proceeding from the cell; cut off the 

 greater part of it by means of the shunt. (Fig. 287.) 



Bring the wires from the cell to the binding screws of the 

 shunt. With the plug placed in the hole between the binding 

 screws, there is no resistance offered by the shunt. The whole 

 current consequently flows through the shunt, none going 

 through the circuit of the galvanometer. The shunt may thus 

 be used as a key, and it will not be necessary to have another 

 key between the galvanometer and the electrodes. If a plug 

 be inserted in the hole marked 1-9, and the plug between the 

 binding screws be withdrawn, such resistance is offered by the 

 shunt, that one-tenth of the total current of cell goes through 

 the galvanometer. Similarly with the plug in the hole marked 

 1-99, instead of in the hole marked 1-9, 1-1 00th goes through 

 the galvanometer; so also with 1-1)99. 



By means of the shunt send l-1000th of the current from a 

 cell through the galvanometer, and mark the direction in 

 which the light travels. Note which screw of the galvanome- 

 ter is connected with the kathode, and which with the anode, 

 and the relation of the direction of travel of the spot of light, 

 to that of the current is known. Most probably it will be 

 found that the light travels in the same way as the current. 



Obs. II. Prepare two non-polari/ablc electrodes with trun- 

 cated ends, or with the plug projecting; connect them with 

 the shunt, using it as a key. 



The plug being iu the shunt and the spot of light at zero, 



