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SCIENCE 



[N. S. Vol. XXVI. No. 676 



as it makes 500 revolutions per second, it 

 would have to generate 200 complete pe- 

 riods for each revolution, so that the space 

 available for the windings and poles for 

 one complete period will be less than one 

 tenth inch, a space into which it is quite 

 impossible to crush the necessary iron and 

 copper to obtain any considerable amount 

 of power. In spite of the small space that 

 we have allotted to each period, as there are 

 100,000 periods per second, the speed of 

 the surface of the moving part works out 

 at over 500 miles per hour. A small al- 

 ternator has been built to give over 100,- 

 000 frequency, but the amount of power it 

 produced was extremely small. Several 

 experimenters have stated lately that they 

 have built alternators giving these high 

 frequencies and a considerable amount of 

 power, but, so far as I am aware, there is 

 no reliable data available as to the design 

 of these machines. 



If it should prove possible to construct 

 alternators for these very high frequencies, 

 we shaU be able to obtain a sufficient num- 

 ber of consecutive oscillations of the cur- 

 rent in the aerial of definite frequency to 

 enable very sharp syntony to be obtained. 

 Not only will this greatly reduce inter- 

 ference troubles in wireless telegraphy, but 

 such alternators will be of the greatest 

 value for wireless telephony. 



The earliest method of producing high- 

 frequency oscillations was proposed by 

 Lord Kelvin, who pointed out that if a 

 Leyden jar or condenser be allowed to dis- 

 charge through a circuit possessing self- 

 induction or electrical inertia, then under 

 certain conditions the discharge of the jar 

 is oscillatory, that is to say, that the elec- 

 tricity flows backwards and forwards in the 

 circuit several times before the jar or con- 

 denser becomes finally discharged. I think 

 that perhaps, the best way to make this 

 matter clear is by demonstrating experi- 

 mentally with an oscillograph the nature 



of the discharge of a condenser, and how 

 it is affected by the resistance and self-in- 

 duction in the circuit. As a mechanical 

 analogy one may look upon the charged 

 condenser as a weight attached to a spring 

 which has been pulled away from its posi- 

 tion or rest. To discharge the condenser 

 we let go the weight and it begins to oscil- 

 late backwards and forwards, and, after 

 making a greater or less number of oscilla- 

 tions, finally comes to rest. The number 

 of oscillations per second will depend upon 

 the strength of the spring and the mass of 

 the weight, which correspond with the ca- 

 pacity and self-induction in our electrical 

 circuit. The number of oscillations before 

 the weight finally comes to rest is deter- 

 mined by the friction which tends to stop 

 - the weight, or by the resistances and other 

 losses in the electrical circuit. 



In practise the aerial conductor acts as 

 a Leyden jar or condenser. It is charged 

 with electricity and allowed to discharge, 

 the current oscillating backwards and for- 

 wards in the aerial during the discharge. 

 In many installations Leyden jars or con- 

 densers are electrically connected to the 

 aerial, so that the oscillations taking place 

 in them are transmitted to the aerial. Any 

 remarks, therefore, that I may make as to 

 the oscillations which may be set up in 

 condensers apply equally well to the oscil- 

 lations in the aerial in wireless telegraphy. 

 For wireless telegraphy it is usual to 

 charge the condenser or aerial by means of 

 an induction coil or an alternator to a very 

 high voltage, and it is allowed to discharge 

 by means of a spark between the two elec- 

 trodes which form the ends, so to speak, 

 of a gap in the electrical circuit. As long 

 as the pressure is low the spark gap is a 

 perfect insulator; when the pressure be- 

 comes high enough the air between the elec- 

 trodes breaks down and a spark passes the 

 gap, becomes conducting, and allows the 

 condenser to discharge. The property of 



