WHEEL COMMUTATORS. 



thence by the metal of the wheel to r 7 , when r* is in contact with 

 any of the metallic parts m of the edge of the wheel, hut will be 

 ^u^pended while it is in contact with the ivory parts i of the edge. 

 If the wheel, being impelled by clock-work, be moved at such 

 a rate that each of the divisions marked m and i shall move tinder 

 the spring in one second, the current will be transmitted and sus- 

 pended also during intervals of one second. It will in fact be sub- 

 ject to a regulated pulsation, the rate of which will be controlled and 

 determined by the horological mechanism which impels the wheel. 



130. In some cases, the motion to be imparted to the wheel is 

 not either regular or continuous. In such cases, it may be moved 

 either directly by hand, or by a strap, or even by clock-work, 

 which is subject to a check which will suspend it at certain 

 positions of the wheel. In all these cases the pulsations of the 

 current in number, length, and continuance, are governed by 

 the motion imparted to the wheel. 



131. As the suspension and transmission of the current are 

 instantaneous upon the breach and re-establishment of the metallic 

 contact of the spring i j and the wheel, there is no practical limit, to 

 the rapidity which can be given to its pulsations. The wheel may be 

 turned, for example, so that 500 divisions of its edge may pass under 

 the spring r' in a second, in which case there would be 250 intervals 

 of transmission, and 250 intervals of suspension in a second. 



It might perhaps be imagined that in so short an interval of 

 time the current could not be stopped or established along the 

 entire length of the conducting wire. It has however been shown 

 that even with the longest continuous wires, practically used in 

 telegraphs, the ten-thousandth part of a second is more than 

 enough either to establish or stop the current. 



132. The intervals of the suspension of the current may be 

 produced by a common toothed- wheel, 



as represented in fig. 50, without ivory 

 or other inlaid non-conducting matter. 

 In this case, a piece of wedge-shaped 

 metal connected with the up line wire 

 is attached to the under side of a 

 wooden lever, while the axle of the 

 wheel is kept in constant metallic con- 

 nection with the down wire. TVhen 

 a tooth of the wheel comes against the 

 metal attached to the lever, metallic 

 contact is established, but when the 

 metal falls between the teeth, and the c < 

 surface of the wooden lever rests on one of them, and the metallic 

 contact being broken, the current is suspended. 



189 



