820 



SCIENCE 



[N. S. Vol. XXVI. No. 07.0 



with the wireless transmission referred to 

 above. 



As an example of the sharpness of tuning 

 obtainable by the spark method the follow- 

 ing test carried out on the Lodge-Muirhead 

 installation at Hythe may be of interest. 



The station at Hythe had to receive mes- 

 sages from Elmers End at a distance of 

 fifty-eight miles over land, in spite of the 

 fact that the Admiralty station at Dover, 

 only nine and one fourth miles distance, 

 was transmitting as powerfully as it could, 

 in order to produce interference, and that 

 the regular communications were going on 

 in the channel between the shipping. It 

 was found possible with a difference of 

 wave-length of 6 per cent, to cut out the 

 interference from the Dover station. 



In the arc method of producing con- 

 tinuous oscillations we employ, as before, 

 a condenser and self-induction; but, in- 

 stead of charging the condenser to a high 

 voltage and allowing it to discharge by 

 means of oscillations which die away, and 

 then repeating the process over and over 

 again, we actually maintain the condenser 

 charging and discharging continuously 

 without any intermission, so that we prac- 

 tically obtain a. high-frequency alternating 

 current in the aerial. 



To impress the difference on your minds, 

 I have an incandescent lamp, which I 

 switch on and off rapidly about ten times, 

 and then after a short time I repeat the 

 same flickering of the light, and so on. 

 The flickering of the light corresponds with 

 the oscillations in the ordinary spark 

 method, and the time spaces between the 

 flickers represent the times during which 

 the condenser or antenna is being charged 

 ready to produce a fresh series of oscilla- 

 tions. In practise we may have as many 

 as, say, a couple of hundred discharges of 

 the condenser a second, and during each 

 discharge we may get, say, ten complete 

 oscillations, each oscillation lasting one 



millionth of a second, if the wave-length is 

 300 meters; thus the total time that the 

 condenser is discharging is only one one- 

 hundred-thousandth of a second, or one 

 five-hundredth part of the interval of time 

 between two successive discharges. My 

 lamp here flickers about five times per 

 second, and makes ten flickers before it 

 goes out ; the total time that it is flickering 

 is two seconds, and the time before it should 

 start to flicker again to correspond with 

 the practical wireless case is therefore 

 1,000 seconds, or rather over a quarter of 

 an hour. If now I represent continuous 

 oscillations, such as are obtained by the 

 arc method with this lamp, I shall simply 

 keep the lamp flickering continuously, and 

 there will be no intervals whatever. 



The arc method of producing continuous 

 oscillations is founded on my musical arc. 

 In order to explain this I must demonstrate 

 some of the properties of the direct-current 

 arc. If I vary the current flowing through 

 the arc very slowly and note the potential 

 difference corresponding with each value 

 of the current, keeping everything else 

 constant, I obtain a curve generally spoken 

 of as the characteristic of the arc. These 

 curves under different conditions have been 

 very thoroughly investigated by Mrs. 

 Ayrton. 



With the carbon arc between electrodes 

 in air the voltage decreases very rapidly 

 when the current is gradually increased, 

 starting from very low values. As the 

 current becomes larger the rate of decrease 

 of the voltage becomes less and less until 

 it is, comparatively speaking, quite small, 

 with a current of ten or twelve amperes. 

 With the arc between metal electrodes simi- 

 lar results are obtained, except that the 

 discontinuity in the curves, called the hiss- 

 ing point by Mrs. Ayrton, takes place at 

 very small currents, generally weU below 

 an ampere. 



