252 THE ELECTRIC TELEGRAPH. 



end of which the other end of the wire was connected. 

 The metal cap was in permanent connection, through a 

 spring pressing upon its periphery, with a terminal screw ; 

 while a similar spring-contact kept the brass cylinder con- 

 nected with another terminal screw forming the ends of the 

 system. The axis of the wooden roller was furnished with 

 a pointer or index which turned with it over a circular dial, 

 and indicated the fractions of turns, whilst a straight bar 

 between the roller and cylinder, graduated correspondingly 

 with the worm of the former, showed the number of whole 

 turns upon it. 



If, in the point where the wire met the brass roller, perfect 

 contact had been made, the length indicated by the rule and 

 index would have represented the resistance ; but this was 

 never strictly the case : there was always a resistance to 

 passage at the point in question, which, being nearly con- 

 stant, had more effect when the length of the wire in circuit 

 was small than when it was great. 



24. Jacobi's .Zs^osfo^. Jacobi, of St. Petersburg, also in- 

 vented a Rheostat, whose purpose, like that of Wheatstone's, 

 was to render the handling of lengths of wire for resistances 

 convenient to the operator. It consisted of a roller of dry 

 wood, in the worm of which, from end to end, a long German- 

 silver wire was wound tightly. One end of the roller was 

 furnished with a metal cap, to which that end of the wire was 

 permanently attached ; the other end of the wire was insu- 

 lated. In front of the roller was fixed a straight round bar 

 of brass, on which a metal jockey wheel, with a groove in its 

 rim, rode over the German-silver wire, pressing upon it suf- 

 ficiently to make a tolerably good contact. The Rheostat 

 was put into a galvanic circuit by means of terminals : the 

 one in connection, through the metal bearing and cap, with 

 the wire, and the other forming one of the supports of the 

 guide-rod. The current passed through the support and cap 

 and through the convolutions of the wire, until it reached 

 the jockey wheel, by which it left the wire. By turning the 

 handle on the axis of the roller, the jockey wheel travelled 

 along the guide-rod, and more or less resistance was intro- 



