54 THE ELECTRIC TELEGRAPH. 



For the purpose of stopping the contact-wheel at its proper 

 place for each of the letters, House employs a key-board like 

 a piano, with twenty-eight keys, representing twenty-six 

 letters of the alphabet, a dash, and a dot. 



The contact- wheel is of brass, four or five inches in dia- 

 meter ; its circumference is divided into twenty- eight equal 

 spaces, alternately indented to the depth of a quarter of an 

 inch, so as to expose on the surface fourteen shallow teeth. 

 A spring of metal, insulated from the contact-wheel and 

 shaft, is placed before the former of these, so that when it 

 revolves, the top of the spring comes in contact with each 

 of the teeth in succession, but has not the power to penetrate 

 into the spaces and make contact there. 



The plan adopted by Wheatstone to hold his current on or 

 to interrupt it, during the reading of a signal, was to stop 

 his commutator opposite an index or pointer at the letter 

 to be indicated. House does this otherwise, with the aid of 

 his piano keys : he puts two rows, of each fourteen pegs 

 on the outside of a cylinder, each peg turning with the 

 cylinder underneath one of the keys of the piano. The 

 latter are held up by springs, and furnished with hooks or 

 cams which, when depressed, catch hold of the pegs of the 

 revolving cylinder and arrest its motion. 



The pegs of successive letters follow each other round the 

 circle in a spiral at distances of one twenty-eighth of the 

 circumference, and therefore, when the cylinder is turned 

 from one letter to another, just so many contacts and inter- 

 ruptions are given as will bring the pointer or wheel of the 

 receiving apparatus round the same distance. 



The receiving apparatus is rather complicated ; it is 

 started by an electro-magnet of very novel construction. 



Above the movable armature, on a common shaft, is a 

 hollow cylindrical slide-valve, in connection with a chamber 

 of compressed air, filled by means of a pump, and supplied 

 with a safety-valve to permit the escape of superfluous air 

 when the pressure becomes greater than is required for work- 

 ing the apparatus. 



The piston moved by the compressed air let into the 



