ANIMAL AND VEGETABLE 237 



position, come to rest in the plane of the magnetic meridian, 

 because very small permanent magnets are affixed trans- 

 versely thereto, and must, consequently, fall into line with 

 the earth's magnetism. The purpose of the controlling 

 magnet is to obtain a position in which it quite neutralises 

 the earth's magnetism. To adjust zero, therefore, a rough 

 approximation to it should be made, before the controlling 

 magnet is in place, by turning the milled suspension pin 

 to the right or left as the case may be but avoiding any- 

 thing approaching a complete turn then putting on the 

 controlling magnet and moving it gently out of the north 

 and south until the reflected spot of light nears the zero of 

 the scale. Further and more delicate adjustments may be 

 made by turning the screw at the back of the pillar, and 

 that operating the ratchet upon the scale-stand. 



Sensibility may be varied by, also very gently, moving 

 the controlling magnet up or down its support. 



Advantage may be taken of the equal number of coils 

 to make the instrument differential. That is to say, by 

 using the two sets of coils separately one current may be 

 sent in one direction and another current in the opposite 

 direction, so that comparison may be made of their respec- 

 tive strengths. If both are exactly equal there will be no 

 deflection, but if one is stronger than the other the spot of 

 light will travel over the scale and indicate the excess. 

 By preliminary experiment the direction of deflection by 

 each current can be determined separately, and in this way 

 the difference of intensity between the two ascertained. 



In experienced hands this galvanometer is as near 

 perfection as anything made by man can be, but, unlike 

 those of the moving-coil type, it is directly affected by any 

 outside vehicle of magnetic or electrical energy. The 

 near proximity of a steel key or even a steel trousers' 

 button is sufficient to cause a movement of the light, and 

 so sensitive is it to induction that it cannot be used 



