of Wheatstone's Rheostat. 



31 



is coiled in a spiral groove upon an insulating cylinder. This 

 is mounted upon the middle of a brass axle of rather more 

 than three times its length. Upon one of the projecting 

 ends of the axle a screw is cut, the pitch of which is equal to 



the distance between the consecutive turns of the wire. The 

 axle revolves in two brass bearings, fixed at a distance apart 

 equal to twice the length of the cylinder ; one of the bearings 

 has an inside screw corresponding with that upon the axle. 

 A flat spring is attached at one end to the base-board of the 

 instrument midway between the bearings ; to the other end 

 is riveted a short copper pin, which is directed perpen- 

 dicularly to the axis of the cylinder and bears upon the spiral 

 wire, being kept in position by a shallow notch cut in its end. 

 One end of the spiral wire is electrically connected with the 

 brass axle, and thence through the screwed bearing and a 

 strip of copper with a terminal upon the base-board. The 

 spring is directly connected with a second terminal. When 

 the cylinder is turned by means of a handle, it travels back- 

 wards or forwards in the direction of its axis, the point of con- 

 tact of the copper pin with the spiral wire remaining fixed in 

 space ; thus more or less resistance is introduced between the 

 axle and the spring, and therefore between the two terminals. 

 In the model exhibited at the meeting of the Physical 

 Society, the length of the cylinder is 3 cm., its diameter is 8 

 cm., and the diameter of the wire, which is of German silver, 

 is 0*5 mm. There are nine turns of wire per centimetre of 

 length, and the greatest resistance is about 10 ohms. Since 

 the instrument is generally used in conjunction with a set of 

 coils, this resistance is for most purposes amply sufficient ; 

 indeed I am inclined to think that it would be on the whole 

 advantageous to use thicker wire and wind it in a coarser 

 spiral, so that the total resistance might be slightly more than 

 one ohm. The rheostat would then be used merely as a fine 

 adjustment*. 



* The best form of cylinder would, I think, be a hollow brass drum 

 covered with a tightly fitting tube of ebonite, 2 or 3 mm. in thickness. 



