COMMUTATORS. 



name of " coinrcrATOES," and are very various in form and 

 arrangement according to the purposes to which, and the condi- 

 tions under which they are applied. Not only do apparatus of 

 this class differ in different countries where telegraphs have been 

 established, but they vary upon different lines, and even on 

 different parts of the same line. Without attempting to follow 

 these endless variations, many of which are quite unimportant, 

 and all of which are mere varieties in the application of the 

 general principles explained above, we shall here confine ourselves 

 to such an illustration of them as will at the same time render 

 intelligible their structure and operation, and convey a general 

 notion of the manner of transmitting and receiving signals. 



111. Let us suppose that around the edge of a disc of ivory, 

 wood, or any other insulating material, are inserted at convenient 

 intervals pieces of metal, B, r, T, D, &c., fig. 47, which we shall 

 call contact pieces, their purpose being to make and break the 

 metallic contact which controls the current. At the back of the 

 disc near these contact pieces are clamps or tighten ing screws by 

 which conducting wires can be attached to them. 



To an axis in the centre of the disc let two metallic hands, A A' 

 be attached, so that they can be turned round the disc like the 

 hands of a clock, but having motions independent of each other. 

 These hands may be supposed to be formed of elastic strips of 

 metal bent at the ends towards the surface of the disc, so as to 

 press upon it with some force : and let one of them move over the 

 other without disturbing it, as the minute hand of a watch moves 

 over the hour hand. Let A." be another similar hand, turning on 

 a centre fixed upon the contact piece E, so that it can be turned at 

 pleasure upon one or other of the contact pieces p or x. 



Xow it is evident that by turning the hands A and A' upon any 

 two of the contact pieces, they will be put in metallic connection, 

 so that a current flowing from either of them will pass by the 

 hands to the other, and in like manner by means of the hand A", 

 either of the contact pieces P or x can be put in metallic con- 

 nection with E. 



112. To convey a general notion of the application of such an 

 apparatus to telegraphic purposes, we shall for example suppose 

 conducting wires connecting the several contact pieces in the 

 following manner : 



1. P, with the positive pole of the battery. 



2. y, with its negative pole. 



3. E, with the earth. 



4. ~c, with the up-line wire. 



5. D, with the down-line wire. 



6. B, with a bell or alarum. 



K 2 179 



