494 Mr. W. Duddell [May 17, 



zigzag winding of the wires on the poles — is of considerable interest. 

 I beHeve it was first put into a high frequency machine by Tesla in 

 1889, and it has been adopted in many modern high frequency 

 machines. 



At about the same date that Sir David Salomons had his machine 

 constructed, Mr. Parsons constructed for Prof. Ewing at Cambridge 

 University a high frequency alternator in which the high frequency — 

 namely, 14,000 — was obtained by using a high speed of 12,000 r.p.m. 



At this stage I would explain a second method of constructing an 

 alternator which differs slightly in principle from the first. If, instead 

 of moving the wires across the faces of the magnets, or moving the 

 soft iron cores on which they are wound, the wires be wound on the 

 poles of the magnets and a piece of soft iron moved past the magnet 

 poles, an alternating current will be produced as before with this 

 diflference— each time the piece of soft iron bridges two poles and 

 thus forms an easy path for the magnetism, the magnetism will in- 

 crease through the coils producing a current in one direction, and as 

 it moves away the magnetism will decrease and produce a current in 

 the opposite direction. It will tlms be evident that we produce a 

 current first in one direction and then in the other as the piece of 

 soft iron passes each individual pole instead of each pair of poles, 

 so that for a given number of poles and a given speed this type of 

 alternator will produce twice the frequency of the previous type. 

 Alternators depending on this principle are usually called inductor 

 alternators. Many high frequency alternators have been constructed 

 on this principle by using 204 polar projections or teeth and a speed 

 of 35,400 r.p.m. I have obtained a frequency of no less than 

 120,000 in a machine I constructed in the laboratories of the Central 

 Technical College, but the output of the machine was microscopic. 



It is of interest to consider what is the practical limit of frequency 

 obtainable in alternators, and this entirely depends on the limiting 

 speed from the point of view of safety at which the material may be run, 

 and on the spacing of the poles. It is quite well known that for every 

 material there is a limiting peripheral speed above which it will fail. 

 With modern steels and good construction, the safe limit is somewhere 

 in the neighbourhood of 300 metres per second. If we assume that 

 we can get a pole and the wire and insulation into 3 mm., the total 

 number of poles per second will be 100,000, and the limiting fre- 

 quency would either be 100,000 or 50,000 according to whether the 

 alternator was of the inductor or the ordinary type. If it were 

 possible to build two discs rotating in opposite directions, having the 

 above peripheral speeds, then the above frequencies would be doubled. 

 The limited width available for the pole and copper leads to great 

 difficulties if any considerable output is required. 



Turning to modern high frequency machines, we have the alternator 

 designed by Mr. Alexanderson, with which frequencies as high as 200,000 

 have been obtained. Owing to the courtesy of Mr. Alexanderson, I am 



