BELGIAN EAILWAY TELEGRAPH. 



will be attracted by a and b, and repelled by 6' and a', and will 

 accordingly move towards the latter until it is stopped by 1 1. 



If the direction of the current be reversed rapidly, suppose, for 

 example, ten times per second, the armature g g' will be made to 

 oscillate ten times per second between the stops 1 1 and f f. 



It is evident that the expedient adopted by Siemens, by which 

 the transmission of the current is arrested by the contact of the 

 armature with one stop and re-established by its contact with the 

 other, might be easily modified so as to reverse the direction of 

 the current by each contact with 1 1 and f t' ; and in that case 

 the telegraph of Siemens would without other change be ren- 

 dered exempt from the defects imputed to it, as well as the French 

 instruments, by Lippens. But M. Lippens, either prevented from 

 adopting this obvious expedient by the patent of Siemens, or 

 giving a preference to the hand commutator for other reasons, 

 has contrived an ingenious commutator worked by hand, by which 

 he reverses the current with the greatest facility, rapidity, and 

 precision. 



203. This is a wheel commutator formed on the principle ex- 

 plained in 129, but there are two wheels such as are there described 

 placed one upon the other upon a common axle, with a disc of 

 gutta percha between them, so that one is insulated from the 



Fig. 79. 



other. The edges of both are divided into a series of conducting 

 and non-conducting arcs, but the position of these relatively to 

 each other is alternate, the conducting arcs of each disc corre- 

 sponding in position with the non-conducting arcs of the other. 



We may imagine the shaded arcs of fig. 79 to represent the 

 conducting arcs of the upper, and the white arcs the conducting 

 arcs of the lower disc, the one, however being separated from all 

 contact with the other by the interposed disc of gutta percha. 

 D 2 35 



