Prof. E. Bouty's Studies on Magnetism. 



Fig. 2. 



85 



The following are the dimen- 

 sions of one of the apparatus 

 which I have employed : — needle 

 A B, length 6 centims., diameter 

 0*2 millim. ; mirror M, diameter 

 8 millims. ; tube T, length 2 cen- 

 tims., diameterO'5 millim.; stick, 

 a square prism of 4 millims. in 

 the side, and 5 centims. length. 



The construction presents no 

 difficulty. The condition most 

 requisite to be realized is, to ren- 

 der the axis of the tube T sen- 

 sibly perpendicular to the axis of 

 the needle AB. For this a gra- 

 duated circle ofpaper is employed; 

 in the centre of the circle the stick 

 of wax is placed with the needle 

 A B already adherent to its lower 

 extremity, and the axis of the 

 needle is made to coincide with 

 the line 0-180°. Then on the 



upper face of the stick the tube T is placed, with a long and 

 stiff steel wire inside of it ; and the axis of the tube is directed 

 so that the eye, placed in the prolongation of the stick, sees the 

 steel wire projected on the line joining 90° and 270°. It is 

 sufficient then to put a lighted match near the base of the tube 

 to slightly melt the wax and fix the tube. Finally another piece 

 of wax, terminated by the nipper c, is fitted on. 



The mirror is set perpendicular to AB by placing the image 

 of A B on the prolongation of its direction. 



Placing. — The placing of the telescope and the scale is done 

 in the usual way*. The distance chosen, of the mirror from the 

 scale, was equal to 4 metres in most of the experiments, and 

 7 metres in some of them. 



For the regulation of the apparatus, a few drops of wax are 

 added, according to need, before or behind A B, so as to bring 

 the image of the scale into the field. The scale is suspended by 

 a system of strings and counterpoises, so that it can be raised or 

 lowered at will by a simple movement of the hand, without the 

 eye needing to leave the ocular of the telescope. It thus becomes 

 very easy, after a few trials, to bring the mirror M into a rigo- 

 rously vertical position; and consequently the bar AB, per- 

 pendicular to the mirror, is itself horizontal. 



* Vide the .writings of Gauss and Weber, or Verdet, Conferences de phy- 

 sique faites a I'Ecole Normale. 



