December 13, 1907] 



SCIENCE 



817 



I have a mechanical model to illustrate 

 how we are able to make our receiving in- 

 struments very sensitive to one frequency 

 and only slightly affected by frequencies 

 which differ but slightly from its proper 

 frequency. 



The transmitter in the model consists of 

 a disc that can be rotated slowly at any 

 speed I like, with a pin fixed eccentrically 

 on its face. This pin can be connected to 

 a vertical wire which moves up and down 

 as the disc rotates. I shall assume that 

 the movement of this wire corresponds with 

 the movement of the electricity in the ver- 

 tical conductor. As a receiving apparatus 

 I have a pendulum, and representing the 

 ether between the transmitter and receiver 

 I have an elastic thread connecting the pin 

 in the disc to the pendulum. 



When I set the disc rotating slowly the 

 elastic thread is alternately stretched out 

 and relaxed and the pendulum is a little 

 affected. If I gradually increase the speed 

 of the disc at one definite speed it will be 

 found that the pendulum is set into violent 

 oscillation, and by observation it will be 

 found that when this is the case the disc 

 makes one complete revolution in exactly 

 the same time that the pendulum would 

 make one complete swing if left to itself; 

 that is to say, that the disc and the pen- 

 dulum make the same number of swings 

 per second or have the same frequency ; in 

 music they would be said to be in tune with 

 each other. If instead of allowing the disc 

 to rotate continuously I allow it to make 

 only half a dozen revolutions, then the 

 pendulum will be affected, but much less 

 strongly. The greater the number of revo- 

 lutions the disc makes up to a certain maxi- 

 mum number the more the pendulum will 

 be caused to swing. 



Instead of starting and stopping the disc 

 I can keep the disc rotating and start and 

 stop the pulls on the elastic thread by 

 moving the pin in the face of the disc in 



and out from the center, which produces 

 a movement which much more nearly cor- 

 responds with the actual current in the 

 vertical wire as used in spark telegraphy. 



It is necessary here to explain the rela- 

 tionship that exists between the wave- 

 length, the frequency and the velocity of 

 propagation of Hertzian waves. The 

 waves travel with, as far as we know, the 

 same velocity as light, namely, 300,000,000 

 meters, or 186,000 miles, per second. Be- 

 tween these quantities we have the relation- 

 ship that the product of the wave-length 

 by the frequency is equal to the velocity of 

 propagation, or, as I have already men- 

 tioned, the velocity of light. 



The wave-lengths which are of practical 

 use in wireless telegraphy at the present 

 time range between 100 and 3,000 meters, 

 though, of course, it is quite possible to use 

 for special purposes wave-lengths outside 

 these limits. The corresponding frequen- 

 cies in practical use are therefore between 

 3,000,000 and 100,000 complete periods per 

 second. We require, therefore, to produce 

 in the vertical conductor alternating or 

 oscillating currents of any frequency with- 

 in this range, and to have a sufficient num- 

 ber of oscillations following one another 

 without interruption to allow of good syn- 

 tony being obtained. 



There are three methods of producing 

 these currents— namely, the alternator, the 

 spark and the arc methods. 



There are great difficulties in the way of 

 constructing an alternator to give such 

 high-frequency currents, and I can best 

 illustrate this by taking an example. Sup- 

 pose that it is required to build an alter- 

 nator to work at the lowest frequency, 

 namely, 100,000 periods per second, and let 

 us assume that we can drive this alternator 

 by means of a turbine at the high speed of 

 30,000 revolutions per minute. This alter- 

 nator could not have a diameter much 

 above six inches for fear of bursting ; and, 



