OX TRANSIENT ELECTRIC CURRENTS. 337 



The twisting' apparatus consisted of a graduated circle provided 

 with a small twisting arm (a), which can he clamped in any position. 

 (See Fig. 1). A stout brass axis passed through the centre of the 

 circle, and the twisting arm was firmly fixed to it. Round the rim 

 of the graduated circle, two grooves were cut. In one of these, two 

 clamps (b) (6) were made to slide. These clamps were capable of 

 being fixed in any desired position by means of screws, and served to 

 stop the motion of the twisting arm at the desired angle of twist 

 when the wire was suddenly twisted. The clamp and the grooves 

 are shewn in Fig. 2, which represents the lateral view of the twisting 

 apparatus. 



The magnetized wire could not of course be near the jnüvano- 

 meter; and being under the necessity of working alone, I was com- 

 pelled to use some device for effecting the twist at a distance. For 

 this purpose, two strings were attached to the twisting arm, and made 

 to slide in opposite directions. The strings after passing the groove 

 were made to slide on two pulleys fixed on the table, on which the 

 whole apparatus rested. The axes of these two pulleys were parallel 

 and their planes perpendicular to the face of the circle. The distance 

 between them was ecuial to the diameter of the circle, and they were 

 fixed in such a position that the strings on leaving the groove and 

 passing down to the pulleys were both vertical. The two clamps (/>) 

 (b) being adjusted so as to stop the twisting arm at the desired posi- 

 tions, the wire was twisted in either direction by simply pulling the 

 proper string. In fact, the mechanism was similar to that of the 

 rudder of a ship. 



The wire under examination had its one end clamped to the ex- 

 tremity (c) of the axis of the graduated circle, being held in position 

 by pressure sustained by a small nut (V), as sufficiently indicated in 

 Fig. '2. Between the clamp and the graduated circle a projecting rim 



