SCIENCE AND PRACTICE. 253 



duced by bringing, in this way, the jockey wheel towards 

 one end or the other of the roller. 



25. Poggendorff's Rheostat. Professor Poggendorff called 

 the instrument which he arranged for the same purpose a 

 Rheocord. Four parallel wires were stretched on a board 

 between terminal screws, the two middle ones being connected 

 permanently together. Between the two on the one side and 

 the two on the other, sliding contacts were introduced, which 

 could be brought to the extremes at each end.- The current 

 from the terminal went therefore through No. 1, crossed over 

 the sliding contact, went down No. 2, crossed to 3, traversed 

 3 as far as the other sliding contact, crossed to 4, and left 4 

 at the terminal on that side. When the sliding contacts 

 were brought down to the bottom, the current passed from 

 terminal to terminal over the contacts without going through 

 any length of the four wires ; whereas, when the slides were 

 at the top, the current had to pass through the whole length. 

 The places of the contacts were read off by their distances 

 from the bottom. These distances being I and I', the resist- 

 ance R, between the terminals, expressed in length, was, 

 therefore 



B=2(Z -M') 



We have spoken of these arrangements in the past tense, as 

 we believe they are one and all superseded by those which 

 follow. 



26. Siemens' Resistance Boxes. A much more handy 

 method of varying the length of the interposed resistance 

 wire is by means of a succession of short circuits between 

 different points of its length, the wire being stationary in- 

 stead of being continually wound and unwound or touched 

 by contact rollers, as is the case with the Rheostats, by which 

 the wire may easily become elongated and hardened, and is 

 always liable to be damaged. 



The method we speak of is best understood by supposing 

 a length of wire (Fig. 129) between the terminals a and b, 

 so arranged that a point c, at the distance a c from the end a, 

 equal to one unit, can be put into direct communication, by 

 means of a short circuit, with the terminal b. A current 



