io8 PHYSIOLOGICAL PHYSICS. [Cha P , x. 



behind. It is long enough, when pushed not quite home, 

 to allow of the needle coming against it when about 

 one-third of the half twist is completed, so that the 

 needle's farther movement is blocked. In front a glass 

 plug may be placed to permit the needle to be seen. 

 As soon, then, as this twisting tendency is observed, the 

 magnet should be slightly removed, till the tendency 

 just disappears and the needle is left just sufficiently 

 under the directing influence of the earth to keep it in 

 the meridian. The instrument will now be found to 

 be very sensitive. When BOTH COILS are to be used a 

 wire must be carried from a binding screw of one to 

 a binding screw of the other ; thus, the binding screw 

 marked 1 of the first coil to that marked 2 of the second, 

 or 3 to 4, one vacant screw of each coil receiving one 

 of the wires conveying the current. To diminish the 

 effect of the current on the needle, the coils may be 

 removed by the sledge arrangement a little way from 

 the copper chamber ; a centimetre scale pasted at the 

 side indicates the distance. When both coils are close 

 over the chamber the most intense effect is obtained. 

 One coil only may be used ; or, to get a differential 

 effect, one current may be caused to traverse one coil, 

 and the current to be compared with it, the other. 



For demonstration purposes, a beam from a 

 lime or electric light, placed at a considerable distance 

 from the boussole, is received on a small plane mirror, 

 and thrown on to the mirror attached to the magnet. 

 The reflected spot is caught on a white scale placed 

 at some distance, 6 to 15 feet, according to the 

 amount of magnification desired. The scale, of 

 course, must be horizontal to the coils. With such 

 an arrangement the author has assisted in showing 

 galvanometer experiments of extreme delicacy to as 

 many as 2,000 people at once. 



For private work a reflected spot of light is 

 not used. At a distance of from 6 to 9 feet from 



