August 22, 1907] 



NA TURE 



427 



The number of possible simultaneous messages depends 

 on the number of octaves there are on the piano used, and 

 on how close together the different notes are which can 

 be used without producing confusion. For instance, it 

 might be quite easy to train someone to distinguish with 

 certainty between " C " and " E," and pick out signals 

 on " C " at the same time that signals are being sent on 

 "E." It is certainly more difficult to do this with two 

 notes that are closer together, say "C" and " D," and 

 still more difficult if the half-tones are used as well. The 

 problem, therefore, in wireless telegraphy is to arrange the 

 receiving apparatus so that it can hear, or perhaps I should 

 say, more accurately, so that it can only see, notes of one 

 definite frequency or pitch, and not be aftected by any 

 other notes, even though of but slightly different pitch. 

 .Another requirement to obtain good working is that we 

 should use as little power as possible at our transmitting 

 station consistent with obtaining enough power in our 

 receiving instruments to work them with certainty. 



I have a mechanical model to illustrate how we are able 

 to make our receiving instruments very sensitive to one 

 frequency, and only slightly affected by frequencies which 

 differ but slightly from iti proper frequency. 



The transmitter in the mode! 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 vertical con- 

 ductor. .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. 



VA'hen 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 pendulum make the 

 same number of swings per second or have the same fre- 

 quency : in music they would be said to be in tune wilh 

 each other. If instead of allowing the disc to rotate 

 continuously I allow it to make only half a dozen revolu- 

 tions, then the pendulum will be affected, but much less 

 strongly. The greater the number of revolutions the disc 

 makes up to a certain maximum 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 centre, which produces a movement 

 which much more nearly corresponds with the actual 

 current in the vertical wire as used in spark telegraphy. 



It is necessary here to explain the relationship 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 metres, or iS(i,ooo miles, per second. Between 

 these quantities we have the relationship that the product 

 of the wave-length by the frequency is equal to the velocity 

 of propagation, or, as I have already mentioned, the 

 velocity of light. 



The wave-lengths which are of practical use in wireless 

 telegraphy at the present time range between too and 3000 

 metres, though, of course, it is quite possible to use for 

 special purposes wave-lengths outside these limits. The 

 corresponding frequencies 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 within 

 this range, and to have a sufficient number of oscillations 

 following one another without interruption to allow of 

 good syntony 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. Suppose that it 



NO. 1973, VOL. 76] 



is required to build an alternator 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 jo,ooo revolutions per ininute. 

 This alternator could not have a diameter much above 

 (1 inches for fear of bursting ; and, as it makes 500 

 revolutions per second, it would have to generate 200 

 complete periods for each revolution, so that the space 

 available for the windings and poles for one complete 

 period will be less than i/io 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 more than 

 500 miles per hour. A small alternator has been built to 

 give more than 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 pow-er, 

 but, so far as I am aware, there is no trustworthy data 

 available as to the design of these machines. 



If it should prove possible to construct alternators for 

 these very high frequencies, we shall be able to obtain a 

 sufficient number of consecutive oscillations of the current 

 in the aerial of definite frequency to enable very sharp 

 svntony to be obtained. Not only will this greatly reduce 

 interference troubles in wireless telegraphy, but such alter- 

 nators will be of the greatest value for wireless telephony. 



The earliest method of producing high-frequency oscil- 

 lations was proposed by Lord Kelvin, who pointed out that 

 if a Leyden jar or condenser be allowed to discharge 

 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 electricity flows 

 backwards and forwards in the circuit several times before 

 the jar or condenser becomes finally discharged. I think 

 that perhaps the best way to make this matter clear is by 

 deinonstrating experimentally with an oscillograph the 

 nature of the discharge of a condenser and how it Js 

 affected bv the resistance and self-induction 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 position 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 oscillations, 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 capacity and self-induction in our elec- 

 trical circuit. The number of oscillations before the weight 

 finallv comes to rest is determined by the friction which 

 tends to stop the weight, or by the resistances and other 

 losses in the electrical circuit. 



In practice the aerial conductor acts as a Leyden jar or 

 condenser. It is charged with electricity and allowed to 

 discharge, the current oscillating backwards and forwards 

 in the aerial during the discharge. In many installations 

 Leyden jars or condensers are electrically connected to the 

 ai^rial, so that the oscillations taking place in them are 

 transmitted to the aerial. Any remarks, therefore, that I 

 mav make as to the oscillations which may be set up in 

 condensers apply equally well to the oscillations in the 

 aerial in wireless telegraphv. 



For wireless telegraphy it is usual to charge the con- 

 denser or aerial by means of an induction coil or an alter- 

 nator to a very high voltage, and it is allowed to discharge 

 by means of a spark between the two electrodes 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 becomes high enough, the 

 air betueen the electrodes breaks down and a spark passes 

 the gap, becomes conducting, and allows the condenser to 

 discharge. The propertv of 'ihe spark gap of passing almost 

 instantaneouslv from a 'condition of being an insulator for 

 electricity to being an extremely good conductor for elec- 

 tricity is' of the utmost value in the spark method of wire- 

 less telegraphv. The more perfectly the spark gap insulated 

 before the discharge takes place, and the more perfectly 

 it conducts after the discharge has taken place, the better 

 it is for our purpose. 



