132 Mr. L. Schwendler on the General Theory 



pelled, to use this relation (equation VI.) as the basis for all 

 subsequent investigations. 



We will therefore suppose henceforth that 



ad-fy = (VI.) 



is rigidly fulfilled for both the stations. 



But as the value of / depends on the position of the key, 

 which during signalling moves from contact 3 to contact 4 and 

 back, the rigid fulfilment of equation (VI.) necessitates at once 

 that 



w+]3=f, (VII.) 



not only for both the contacts 3 and % but also for all the in- 

 termediate positions of the key. Thus, supposing that w + /3 =/, 

 i. e. the resistance from contact 4 through battery to earth equal 

 to the resistance from contact 3 to earth, a key constructed in 

 such a way that contact 4 is not broken before contact 3 is made, 

 and that contact 3 is not broken before contact 4 is made, would 

 fulfil the required condition entirely. Keys of this kind can be 

 easily enough constructed. It is true that in any such key there 

 will be always a moment when the contacts 3 and 4 are simul- 

 taneous, and when therefore the resistance to earth is not/, as 



f 



it ought to be, but only ^. Considering, however, that the time 



(luring which this error lasts is very small compared with the 

 time it takes to make a signal, its disturbing effect will never 

 be appreciable in practice ; i. e. p will remain sensibly constant 

 during the time the key is moved to produce a signal. 



There will be no practical difficulties connected with the ful- 

 filment of equation (VII.), and therefore also none with the ful- 

 filment of equation (VI.); for &, the internal resistance of the 

 signalling-battery is the only quantity which of itself can alter 

 in time. However, this variation of /3 for any efficient form of 

 signalling-battery being invariably steady and small, it will be 

 always possible to neutralize its action in time by a simple read- 

 justment of w. 



If Leclanche's cells are used, or well prepared Minotti's, a 

 weekly adjustment of w should be sufficient. The measuring 

 of /3 will always be an easy matter*. 



* My friend Mr. R. S. Brough suggested the following very simple me- 

 thod for keeping 



w+P=f. (VII.) 



Insert a small galvanoseope in the branch b, for which balance is estab- 

 lished with respect to the received current, i. e. 



ad-fg = i (VI.) 



Now note the deflection on the galvanoseope when both stations are 



