CH. XII.] 



THE GALVANOMETER. 



141 



knowledge, and among the names of modern physiologists associated 

 with it must be particularly mentioned 

 those of Du Bois Reymond and Her- 

 mann. 



Before we can study these it is, 

 however, necessary that we should 

 understand the instruments employed. 



The Galvanometer. The essen- 

 tial part of a galvanometer is a mag- 

 netic needle suspended by a delicate 

 thread ; a wire coils round it ; and if 

 a current flows through the wire, the 

 needle is deflected. Suppose a man to 

 be swimming with the current with his 



Fig. 162. Reflecting galvanometer. (Thomson.) 

 A. The galvanometer consists of two systems 

 of small astatic needles suspended by a fine hair 

 from a support, HO that each set of needles is 

 within a coil of fine insulated copper wire, that 

 forming the lower coil being wound in an opposite 

 direction to the upper. Attached to the upper 

 set of needles is a small mirror about ] inch 

 in diameter ; the light from the lamp at B is 

 thrown upon this little mirror, and is reflected upon 

 the scale on the other side of B, notshown in figure. 

 The coils u I are arranged upon brass uprights, 

 and their ends are earned to the binding 

 screws. The whole apparatus is placed upon a 

 vulcanite plate capable of being levelled by the 

 crew supports, and is covered by a brass-bound 

 glaan shade, the cover of which is also of brass, 

 and supports a brass rod l>, on which moves a 

 weak curved magnet m. C is the shunt by means 

 of which the amount of the current sent into the 

 galvanometer may be regulated. When in use the 

 scale is placed about three feet from the galvano- 

 meter, which is arranged east and west, the lamp is lighted, the mirror is made to 

 swing, and the light from the lamp is adjusted to fall upon it, and it is then regulated 

 until the reflected npot of light from it falls ui>on the xero of the scale. The wires from 

 the non-jxilarisahle electrodes touching the muscle are attached to the outer binding 

 crews of the galvanometer, a key intervening for short circuiting, or if a portion only 

 of the current is to pass into the galvanometer, the shunt should intervene as well with 

 the appropriate phi); in. When a current passes into the galvanometer the needles 

 and, with them, thn mirror, are turned to the right or left according to the direction of 

 the current. The amount of the deflection of the needle is marked on the scale by the 

 spot of light travelling along it. 



