ELECTRO-MAGNETISM. 



41 



\vire, by reference to a small graduated 

 circle immediately below it, in the upper 

 side of the cover. All these parts are re- 

 presented in the vertical section,//^. 78. 

 The other parts of the apparatus, as far 

 as relates to the coils of wire which 



Fig. 79, 



Fig. 78. 



_ ^..n ^ n 



^iiiiiiliiii 



encircle the needle, are similar to those 

 of the former instrument : excepting 

 that the wires in the middle of the upper 

 part of the coil must be separated a 

 little, in order to leave an opening for 

 the free passage of the thread that sup- 

 ports the needle. A graduated circle, 

 equal in diameter to the length of the 

 needle, is placed, as in the former case, 

 immediately below the needle. The 

 compass, wire, and card, are enclosed in 

 a box, in order to secure them from the 

 agitations of the air; and the cover, 

 from the middle of which the upright 

 tube rises, should be of glass, in order 

 to allow of our seeing the position of the 

 needle. 



(116.) Mr. Ritchie has lately proposed 

 a torsion galvanometer, in which he em- 

 ploys a thread of glass as the material 

 for suspending the needle*. Fig. 79 is 

 a vertical section of his instrument, of 

 which he gives the following descrip- 

 tion : Take a fine copper wire, and 

 cover it with a thin coating of sealing- 

 wax. Roll it about a heated cylinder, 

 an inch or two in diameter, ten, twenty, 

 or any number of times, according to 

 the delicacy of the instrument required. 

 Press together the opposite sides of the 

 circular coil, till they become parallel, 



* Philosophical Transactions for 1830, p. 218. 



and about an inch, or an inch and a half 

 long. Fix the coil in a proper sole, and 

 connect the ends of the wires with two 

 small metallic cups, for holding each a 

 drop of mercury. Paste a circular slip 

 of paper, divided into equal parts, hori- 

 zontally on the upper half of the coil, 

 and having a black line drawn through 

 its centre, and in the same direction with 

 the middle of the coil. Fix a small 

 magnet, made of a common sewing- 

 needle, or piece of steel wire, to the 

 lower end of a fine glass thread, whilst 

 the upper end is securely fixed with 

 sealing-wax in the centre of a moveable 

 index, as in the common torsion balance. 

 The glass thread should be inclosed in a 

 tube of glass, which fits into a disc of 

 thick plate glass, covering the upper 

 side of the wooden box, containing the 

 coil and magnetic needle. 



(117.) This instrument enables us to 

 estimate the comparative intensities of 

 currents of electricity circulating along 

 the wires of the coil. For this purpose, 

 the needle is to be placed directly above 

 the meridian line drawn on the paper 

 circle, and consequently directly above, 

 and in the direction of the wires form- 

 ing the upper side of the coil. As soon 

 as a current of electricity is made to cir- 

 culate along the wires, the needle will of 

 course be deflected. The glass thread 

 must then be twisted, by turning the 

 index, until the needle is brought to its 

 former position ; and the number of 

 degrees of torsion must be noted. A 

 similar experiment may next be made 

 with another current, the thread having 



