CH. XII.] 



THE GALVANOMETER 



135 



on the needles ; the multiplication of the effect of a weak current in 

 this way is accomplished in actual galvanometers by many hundreds 

 of turns of fine wire. 



Fig. 162 illustrates the best galvanometer: that of Sir William 

 Thomson (now Lord Kelvin). It is called 

 a reflecting galvanometer, because the ob- 

 server does not actually watch the moving 

 needle, but a spot of light reflected on to a 

 scale from a little mirror, which is attached 

 to and moves with the needle. A very 

 small movement of the needle is rendered 

 evident, because the movement of the spot 

 of light being, as it were, at the end of a 

 long lever namely, the beam of light, 

 magnifies it. 



Fio. 102. Reflecting galvanometer. (Thomson.) A. The galva- 

 nometer consists of two systems of small astatic needles 

 suspended by a fine hair from a support, so that each set 

 of needles is within a coil of tine 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 J 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, not shown in figure. 

 The coils u I are arranged upon brass uprights, and their 

 ends are carried to the binding screws. The whole appar- 

 atus is placed upon a vulcanite plate capable of being 

 levelled by the screw supports, and is covered by a brass- 

 bound glass shade, the cover of which is also of brass, and 

 supports a brass rod 6, 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 

 galvanometer, 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 spot of light from it falls upon the zero 

 of the scale. The wires from the non-polarisable electrodes 

 touching the muscle are attached to the outer binding 



screws 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 

 plug in. When a current passes into the galvanometer the needles and, with them, the mirror, 

 are turned to the right or left according to the direction of the current. The amount of the deflec- 

 tion of the needle is marked on the scale by the spot of light travelling along it. 



Non-polarisable Electrodes. If a galvanometer is connected 



