THOMSON'S ASTATIC GALVANOMETER. 



187 



m 



(a.) Place the galvanometer (Fig. 78) upon a stand where 

 it is unaffected by vibrations e.g., on a slate slab fixed into 

 the wall at a suitable height, 

 or on a solid stone pillar fixed 

 in the earth, taking care that 

 no iron is near. 



(6.) Place the galvanometer 

 so that it faces west i.e., with 

 the plane of the coils in the 

 magnetic meridian, the mag- 

 netic meridian being ascer- 

 tained by means of a magnetic 

 needle. As the galvanometer 

 is a differential one, to convert 

 it into a single one, connect 

 the two central binding screws 

 on the ebonite base, by means 

 of a copper wire. 



(c.) By means of the three 

 screws attached to the ebonite 

 base, level the galvanometer. 



(d.) Allow the mirror at- 

 tached to the upper needle to 

 swing freely. Take off the 

 glass cover and steadily raise 

 the small milled head on the 

 top of the upper coils, which 

 frees the mirror. Replace 

 the glass shade. 



Fig. 78. Sir William Thomson's 

 reflecting galvanometer. u, 

 Upper, I, lower coil; s, s, 

 levelling screws; m, magnet 

 on a brass support, &. 



(e.) Place the scale (Fig. 79) 

 also in the magnetic meridian 

 and 1 metre from the mirror, 

 taking care that it is at the proper height. An improved 

 form of scale with several adjustments has recently been 

 introduced. Instead of a mere slit in the scale, it is better 

 to fix in it a thin wire, and by means of a lens of short focal 

 distance to bring the image of the wire to a focus in the 

 middle of the illuminated disc of light reflected from the 

 mirror upon the scale. 



(f.) Light the paraffin, lamp, darken the room, and see 



