SCIENTIFIC AND TECHNICAL LABOURS TO 1860. 227 



the slips. Wheatstone made good use of my three- 

 key-puncher in 1858 for his electro-magnetic express- 

 writer, without however naming the source whence 

 he derived it. 



The signalling service of the railways, with which 

 our firm had from the first been particularly occupied, 

 brought further problems. On all the German railway- 

 lines ringing -apparatus had to be set up, which on 

 the departure of a train from a station should give 

 audible bell - signals for the whole distance. The 

 mechanician Leonhardt had already provided such 

 gong -apparatus for the Thuringian line, but they acted 

 imperfectly, as it was difficult to maintain in good 

 condition the large galvanic batteries, which were 

 required at the stations for setting the apparatus to 

 work. It was an obvious idea to employ magnetic 

 inductors instead of batteries, but the magneto-induction 

 machines, known up to that time, of Saxton and Stohrer 

 were not suited for the purpose. We now constructed 

 a new kind of such inductors, which worked admi- 

 rably, and afterwards entirely superseded all other 

 constructions. The essential feature of our inductor 

 was the employment as a rotating bar of an iron cylin- 

 der, which was provided with deep opposite grooves, 

 forming a channel for the reception of the coil of 

 copper wire. From the form of its iron cross -section 

 this bar received the name of the double T arma- 

 ture; in England it is known as the Siemens' arma- 

 ture. The steel magnets, hollowed out at the end, 

 which surrounded the rotating cylinder, could be set 



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