352 S1JC. 10. ELKCTBIC1TF. 



c. Wheel key. A constant current is maintained on the line, and signals 

 are made by depressing the key and thus reversing the current. A switch is 

 used for making the necessary alterations to the connexions for sending and 

 receiving. 



1528. Plunger Signalling Key, used by the London District 

 Telegraph Company. 



This key is used instead of the drop handle key of Cooke and Wheatstoue's 

 instrument, or the two-pedal key in Highton's instrument. An ordinary 

 single-needle indicator is used in connexion with it. 



1538. Whitehouse's Relay, 1854. 



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. 



1534. Varley's Mill, 1855. Used by Electric Telegraph 

 Company. 



This contrivance was used in connexion with relays and translators. 

 When a current passes through the coils the armature is attracted rapidly 

 in the usual way, but when the current ceases the armature returns slowly to 

 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. (See 1566.) 



1535. Andrews' '* Pump Helay 5 > or " Spacer, 33 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, in cases where the double-current key 

 (1562) could not be used. 



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

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

 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. 



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

 Kingdom Telegraph Company. First form. 



