HISTORY AND PROGRESS. 83 



lower part of the depressed key. Motion is imparted to 

 the axis of the pointer and carrier arm from the handle 

 by means of a bevilled wheel, which engages with a pinion 

 fixed to the axis carrying the armature of the electro- 

 magnet. The proportion is so adjusted that, for every 

 current induced in the coils, the pointer shall advance the 

 distance of one letter on the dial. If, therefore, the pointer 

 and arm start freely from zero when the handle is turned 

 and any key, as D for example, depressed, the armature is 

 rotated, producing alternate electric waves whilst the pointer 

 will pass over A B and c respectively. Arriving at D, the 

 carrier-arm comes into contact with the depressed key and 

 cuts off the passage of the subsequent currents until another 

 key is depressed, by which the carrier-arm is released and 

 travels on with the pointer. 



The face of the indicator is divided into thirty equal 

 spaces exactly similar to the dial of the communicator, with 

 a double circle of letters and numerals. On an axis in its 

 centre is a pointer like the minute hand of a watch, to 

 which motion is given by a small escapement-wheel with 

 fifteen teeth. Two magnetic needles, or bars, fixed to an 

 axis, lie parallel between two small electro-magnetic coils 

 with soft iron cores, and are so arranged that, when currents 

 pass through the coils and magnetise the cores, the latter 

 exercise mutual attractions and repulsions on the poles or 

 extremities of the magnet-needles, imparting a backward 

 and forward motion to their axis. The scape- wheel is 

 carried by a short vertical arm fixed to the end of this 

 axis, and is rotated by working to and fro against stops or 

 pins. 



The electrical connections of the apparatus are simple. 

 The coils of the communicators and indicators of all the 

 apparatus are connected up in a common circuit. When 

 the coils of one of the communicators are turned round 

 by means of the handle, if the pointer is free to move 

 round the dial, a current traverses the line at every letter 

 which the pointer passes over, moving the hands of the 

 indicators correspondingly; but as soon as the carrier-arm 



