552 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK v. 



We will now describe the manipulator of Wheatstone's single 

 needle telegraph. As appears on the right of Fig. 350, which shows 

 the back of the apparatus, the galvanometer G is placed in the centre 

 of the vertical line as represented on the front face in the same figure. 

 The indicating needle is mounted on the same axis as the magnetized 

 needle of the galvanometer. They are also both magnetized, forming 

 a compensating or astatic system as in No bill's galvanometer (see 

 Forces of Nature, book vi). What constitutes in reality the mani- 

 pulator or commutator is situated below T the galvanometer. It consists 

 of a boxwood cylinder supported on two metallic bearings on the 

 axis of the outer handle, and movable like it to right or left. On the 

 outside this cylinder is covered by two metallic bands which are insu- 

 lated from each other. The bearing D is in constant contact with the 

 spring E, and also with the band n. Two metallic points ~b and V rise 

 from each of the bands and, according to their position, come one 

 against the spring K, the other against spring u. The band ra is in 

 permanent contact with the spring K". At M is seen a metallic rod 

 armed laterally with two points which touch in a and a, according 

 to the position of the needle, either the spring u'; or the spring u. 

 Lastly the galvanometer wires are joined to the two pieces z and z' 

 which are themselves united, the first with the end L of the line wire, 

 the second with the springs u' K' and the wire of the positive pole of 

 the battery ; on the other hand, the springs K and u are joined to 

 the earth-wire T, and K" to the negative pole N of the battery, 



This being given, imagine the handle of the manipulator vertical. 

 In this case, the points b and b' are themselves vertical, and the 

 metallic bands of the cylinder remain insulated ; the current from the 

 battery cannot pass from one to the other, nor in consequence enter 

 the galvanometer wire& 



Suppose the handle turned to the right this is the case repre- 

 sented in Fig. 350. The two points I and b' press against the springs 

 K and u 7 , taking the latter out of contact with the piece M. The 

 current will then follow the path marked by the series of letters 

 corresponding to the different pieces of the manipulator in the 

 following order P R D n V z' G z L ; the current thus coming from the 

 line wire after having deflected the upper point of the galvanometer 

 needle in the sender to the right, pursues its course, enters the 

 fe:eiving apparatus and deflects the needle of its galvanometer in 



