682 Prof. J. Perry on 



of 11 per cent, per mile. It is to be observed that notes of 

 frequencies 1 : 2 are attenuated in the proportion 1 : s/%* 

 This is probably good, because it is known that the graver 

 notes of speech need to be of greater intensity for distinct 

 hearing. But the changes in phase are also as 1 : \/% 

 whereas for good hearing they ought to be as \J2 : 1. 



Using this cable with no contrivances it is found in practice 

 that there is good telephonic speech to a distance of 40 miles. 

 When 9 = 5000 a current 1 is reduced to 0*015 or -^ of 

 itself in 40 miles. If ^=6000 (a frequency of 960) a 

 current of 1 is reduced to 0*01 with a lag of 263°; whereas 

 if 9 = 4000 (a frequency of 640) a current 1 is reduced only 

 to 0*0238, but it has a lag of 215°. In fact the two lags in 

 actual time are as 22 to 29. If, then, we find such attenua- 

 tions and lags in any case we may infer that good telephone 

 speaking is possible. 



II. I will first give examples of a tuned circuit, that is, very 

 perfect transmission of one musical note oE frequency 800. 

 Keeping to the contrivance which has two condensers in 

 series, each of capacity K in the line and an inductance leak 

 of L henries to earth from the point between the condensers, 

 it is easy to write out the general expression lor P in terms 

 of m and r x and r 3 (at all events when ma is small), and it may 

 be studied. Now for any particular m find the values of 

 i\ and r% which will cause P to be nearly 1 (0°) and there- 

 fore C to be nearly 1 (0°). I have already explained that 

 we have no right with the above formulae to get a really 

 constant current like tins, but we have a right to approach it 

 very nearly. 



If m = l mile, then K = *5917 microfarad and L = *8324 

 henry. If m — 2*755 miles, K = *2273 microfarad and 

 L = *3692 henry. Even when m is as much as 5*51 miles, 

 a very considerable distance for such a cable, I find that I 

 can use values of K and L which cause the attenuation to be 

 comparatively small. 



We see that by means of contrivances placed at con- 

 siderable distances apart we can tune a long circuit so that 

 it will transmit a current of one frequency, just as if the 

 lino were a mere Ohmic resistance. 



I do not know whether practical men see great value in 

 having this power. It may be important to inventors. It 

 is well, however, to say that when we have such a line, unless 

 the contrivances are rather close together, the circuit is 

 really a tuned one, tuned to a particular frequency and not 

 good for the transmission of other frequencies. 



