Chap, x.] THE ACCESSORY MAGNET. 107 



on a solid stone foundation, is best. On such a sup- 

 port the boussole is so placed that the axis of the coils 

 is perpendicular to the magnetic meridian. In this 

 position the ring, being non-astatic, will place itself 

 so that its sides will point north and south, and the 

 mirror will face the east. 



To render the needle astatic the arrange- 

 ment shown in Fig. 56 is used. It is called Hauy's bar 

 (Der Hauy'sche Stab), and the arrangement in Fig. 56 

 is that of Du Bois. It consists of a magnet, the 

 ACCESSORY MAGNET m, placed in the magnetic meri- 

 dian, and therefore horizontal to the needle. Its 

 north pole should be pointing north, as is that of the 

 needle. It is supported on the bar B, which is 

 directed perpendicular to the coils, and in a line with 

 their axis. The magnet can slide in its support up and 

 down the bar, which is divided into centimetres for 

 measuring the extent of movement. Further, one end 

 of the magnet is caught between a spring and a 

 screw. The screw may be turned by P 1 , so that the 

 magnet can be moved from the spring end on the 

 other end, so as to form an angle with the plane of 

 the coils. By means of the pulley arrangement P 2 

 this angular movement can be effected by the ex- 

 perimenter seated at a distance. The galvanometer 

 then being placed, the accessory magnet is fixed on its 

 bar, by a clamp to the shelf, almost under the end of 

 the mahogany stand of the galvanometer. The magnet 

 is first put on the end of its bar, and is then slowly 

 moved down it. As it approaches nearer the boussole 

 it gradually neutralises the earth's action. The 

 moment the position of neutralisation is crossed the 

 needle swings round so as to place its opposite poles 

 over against the poles of the magnet. It would make 

 in this movement a full half twist on its fibre. To 

 prevent this being accomplished one of the brass 

 plugs should be put in at the opening of the chamber 



