WIEELESS TEANSMISSION OF ENERGY THOMSON. 259 



daylight effect, as pointed out by Fessenden, is much less with the 

 lower frequencies, such as 100,000 per second as compared with 

 GOOjOOO or 800,000 waves. Consequently there is not the same great 

 difference in strength of signals between night and day work with 

 such lower frequencies. Moreover, frequencies of 100,000 or even 

 200,000 are capable of being generated directly by high-speed high- 

 frequency dynamos with the added advantage that the waves sent 

 out are maintained at their full amplitude and are not, as with waves 

 produced by spark discharges, subject to damping or decay from 

 maximum to zero after a few oscillations. 



Whatever the nature of the waves sent out, there is in all cases the 

 need of an exceedingly sensitive apparatus for converting the slight 

 electric effects upon the receiving antenna into signals. The origi- 

 nal apparatus of Marconi included the Branly coherer, used by 

 Lodge in Hertzian wave transmission as a detector. It is indicated 

 in figure 2G at A', with its battery and sounder magnet M. The re- 

 ceiving antenna discharge in passing to earth broke down the in- 

 sulation of the filings of the coherer, so that the local battery cur- 

 rent could pass in the 



circuit, including a mag- pi*25. Fi^.26. Fi^.27. 



net M and so record the i 

 signal. The liquid bar- 

 retter of Fessenden, the \ ; 

 various forms of recti- 

 fjdng crystal detectors j_ 

 and magnetic detectors, c s , 



have been extensively j J ■ ^ 



used. Our time does not J_^ JL ^ 



permit a detailed de- I — I L_l 



scription. Figure 25 in- 

 dicates at C a crystal detector rectifying the impulses from antenna 

 A so lis to work a high-resistance telephone receiver 7", to which the 

 operator listens. Figure 27 shows the same apparatus, but connected 

 inductively to the antenna circuit by a transformer, 

 reaching the telephone T was such as to produce a low note, the 

 signals were easily drowned by extraneous noises or induced effects. 

 He found that the human ear reached a maximum of sensitiveness 

 at about 900 waves of sound per second, so that the signals were 

 heard distinctly when otherwise they would have been missed. This 

 is the meaning of the substitution of dynainos of about 500 cycles for 

 exciting the wireless antenna in place of the ordinary machines of 

 lower frequency. 



The problem of wireless telephony- has attracted attention for a 

 number of years past. I well remember witnessing some of the 



HlHh 



Ui>^ 



