250 THE ELECTRIC TELEGRAPH. 



the scale, and lessening its distance. For all measurements 

 in which the instrument serves as a galvanoscope, as in 

 Wheatstone's bridge, PoggendorfFs compensation method of 

 comparing electro-motive forces, &c., and when the readings 

 are not very different in value, it must be confessed that this 

 galvanometer is much to be preferred. 



Mr. Becker has given it a very convenient form, by fixing 

 the coil in the back of a brass barrel or cylinder, in the front 

 of which a glass plate enables the interior to be seen from 

 before, and prevents dust and currents of air getting to the 

 needle. 



In the centre of the coil is suspended, by a fine cocoon 

 fibre, in a frame, a small silvered mirror* of microscope glass, 

 between one- eighth and one- fourth of an inch diameter. A 

 little magnet, made of a piece of thin watch-spring, is fastened 

 to the back or silvered side of the glass, and, being magne- 

 tised, operates as the needle of the system. Above the brass 

 barrel a vertical rod carries a curved permanent adjusting 

 magnet, and a rack and pinion enables the latter to be turned 

 round horizontally to bring the point of light to any part of 

 the scale which may be desired. 



The adjusting magnet is elevated or depressed on the 

 vertical rod for the purpose of increasing or decreasing the 

 directing force upon the magnet needle. When the ends of 

 the adjusting magnet coincide with the poles of the earth's 

 magnetism, it adds to the directive force of the latter, and 

 the instrument becomes proportionally unsensitive. The 

 magnet may, however, be turned round so as to oppose the 

 directive force of the earth, and in this position be lowered 

 towards the mirror, until it very nearly neutralises the earth's 



* The process of depositing metallic silver upon glass is as follows : (A) 

 Dissolve 10 parts of nitrate of silver in 50 parts of water, and neutralise with 

 (about) 6 parts of liquor ammonia ; add to this a solution (B) of 1 part of 

 tartaric acid in 4 parts of water, and dilute the whole (A -f- B) with 500 parts 

 of water. The things to be silvered should be placed conveniently in a vessel, 

 the solution poured in, and then put away in a quiet place for a few hours, at 

 a temperature of from 40 to 50 C. When silvered they may be washed 

 by a gentle stream of water, dried, and varnished with a solution of amber 

 in chloroform. 



