1912] on High Frequency Currents. 497 



frequency of 200,000 has been obtained by Mr. Alexanderson. It 

 is iiitci'csiiiii; to note that wlien running at this very high speed 

 the gyrostatic action is so considerable that a long flexible shaft has 

 to be used so that the disc spins in its own plane like a top. 

 Auxiliary bearings are provided close to the disc, which, while allow- 

 ing sufficient freedom for the disc to spin in its own plane, yet 

 prevent excessive bending of the shaft when the disc passes through 

 its different critical speeds when being speeded up. To give some 

 idea of the high speeds at which these discs run I may mention that 

 tlie speed of the edge is some 12 miles per minute, so that if they 

 could be used as wheels for a motor car, the journey to Brighton 

 would only occupy about 4| minutes. 



A iiigh frequency alternator on quite a different principle is due 

 to Dr. (loldschmidt. The principle of the machine is at first sight a 

 little difficult to understand. I have mentioned in Sir David 

 Salomon's early machine that the two discs forming the field and 

 armature were rotated in opposite directions. It is possible, how- 

 ever, to prodnce a rotating field electrically without the disc moving 

 at all. \yhen this is accomplished the frequency of the machine 

 depends as befoi'e upon the relative speed of the rotating field and 

 the armature. 



Considering for simplicity a machine having only two poles so 

 that it gives one complete cycle per revolution, and suppose I supply 

 the fixed part or stator with an alternating current of KM) freijuency 

 and so connect it that it will produce a rotating field (we will assume 

 the alternating current to be three phase for the sake of simplicity), 

 if now I rotate the rotating part in the opposite direction to that of 

 the rotating field I shall have induced in the rotor an alternating 

 current whose frequency will depend on the relative speeds. For 

 instance, if the speed of the rotor is such that with a stationary field 

 it would give 100 frequency, then as my field is actually rotating in 

 the opposite direction at 100 frequency the actual frequency pro- 

 duced will be 200 per second. Using this 200 per second frequency 

 by means of another machine running at the same speed one could 

 increase it to 300, and so on indefinitely. This would, however, 

 lead to a very complicated machine, and Dr. Goldschmidt has arranged 

 matters so that instead of requiring a number of machines tlie 

 successive additions to the frequency can all go on in the same 

 machine. 



The simplest way to explain this is to suppose that the alternating 

 current at 200 frequency obtained from the rotor is passed back into 

 the stator of the same machine so as to produce a rotating field of 

 200 frequency without in any way interfering with the original 

 rotating field of 100 frequency. This rotating field of 200 frequency 

 will induce an alternating current of 300 frequency in the rotor, 

 which could in turn be transferred back to the stator and produce a 

 rotating field having 300 frequency, and so on. 



