304 SEC. 10. ELECTRICITY. 



its normal position, by the intervention of a wheel, pinion, and fly, thus 

 lengthening the contact between the line lever and the spacing battery. This 

 is now effected by utilising the extra current from the magnet. 



1535. Andrews' '* Pump Belay or " Spacer," used by 

 the United Kingdom Telegraph Company. 



This is a contrivance to produce a similar result to Varley's mill. A 

 loosely fitting piston with a ball valve moving in a cylinder filled with oil 

 is used instead of the fly. 



1536. Zinc Sender, used by Electric Telegraph Company. 

 Used for sending a short reversal after each signal in the Morse system, to 



assist in discharging the line wire. 



The coils of this apparatus are wound with fine wire of high resistance, and 

 are placed as a " leak " or derived circuit on the line wire at the sending end ; 

 its. action is identical with that of a polarised relay. A single current Morse 

 key is used. When this is depressed a portion of the current passes through 

 the coils and moves the tongue of the relay over in contact with the screw 

 stop which is connected to the reversing battery, the other pole of this 

 battery being in connexion with the earth. 



The back stop of the Morse key is in permanent electrical connexion with 

 the tongue of the relay, and thus, when the key is raised and its lever comes 

 into contact with the back stop, a reverse current will pass out to the line ; 

 but a portion of this reverse current also passing through the coils of the 

 zinc sender will immediately move the tongue to the opposite side, the stop 

 against which it rests being in connexion with the relay of the receiving 

 apparatus. Thus after each reversal following a marking current the 

 instrument is in a position for receiving, and the receiving station can stop 

 the sending station during transmission. A smaller battery than the sending 

 one is used for the reversal. A spring is fixed on the tongue on the battery 

 side to lengthen the contact. 



1537. Andrews' Relay, for Hughes' Type Printing Instru- 

 ment, 1868, used by the United Kingdom Telegraph Company. 



This relay is adapted for relaying the short currents required for working 

 the Hughes' instrument ; its peculiarity consists in the relayed currents being 

 of equal length, and independent of the length of the line current. 



1538. Whitehouse's Belay, 1854. 



In this relay a small permanent horseshoe magnet oscillates between the 

 pole pieces of an electro-magnet. The adjustment is effected by the attraction 

 of another small permanent magnet, instead of the spiral spring generally used. 



1539. Simple Electro-Magnetic Belay, used by the United 

 Kingdom Telegraph Company. 



The simplest form of relay, consisting of an electro-magnet ; its armature 

 is attached to one end of a lever, the other end playing between two limiting 

 stops, the local battery circuit being closed when the armature is attracted. 

 A relay similar in principle was used with Bain's Chemical Marking Tele- 

 graph on the long lines of the Electric Telegraph Company. 



1540. Belays used by the Electric Telegraph Company. 

 Earliest form of relay with an inducing magnet. Superseded by 

 No. 39, 1855. 



The coil is wound on a reel of soft iron, upon each end of which a 

 hollow " casing " or cap of the same material is fitted, almost completely 



