1012] on lUijh Froqiinnrij Currents. 493 



south pole a cun-ent will be produced in the opposite direction, so 

 that we have here ((uite a simple means of producing a current which 

 flows alternately in the wire in opjwsite directions — that is to say, an 

 alteniatinu: current. If a number of mas^-nets are arranged round 

 tiie machine with their poles alternately north and south, and a wire 

 is caused to move past them, an alternating current will be produced, 

 the frequency depending upon the number of magnets and on the 

 number of revolutions the machine makes per second. For instance, 

 if we have 8 magnets, each of which has, of course, two poles, and if 

 the machine makes 6 revolutions per second, then the frequency will 

 be 4S. It will be at once obvious that if we wish to generate a high 

 frequency with a machine such as this, we can obtain it either by 

 using a large number of magnets or a high speed of rotation, or both. 



In a little special machine which I have here there are 15 

 magnets — that is to say, 3() poles alternately north and south — and it 

 runs at 8000 revolutions per minute or 133:^ revolutions per second, 

 so that frequency in this case is 2000. The peripheral speed of this 

 machine is just over 3 miles per minute — that is to say, the rotating 

 part would roll down to Brighton in about 1 7 minutes. From this 

 example it will be pretty obvious that if I attempt to obtain high 

 frequencies — that is to say, frequencies well above 10,000 — I shall have 

 to run either at enormous speeds or have a very large number of 

 poles. One of the very earliest high frequency machines ever con- 

 structed in this country, and following shortly after Tesla's high fre- 

 quency machine, was made by Messrs. Pyke and Harris for Sir David 

 Salomons, and owing to the courtesy of Messrs. Isenthal, who now 

 own the machine, I am able to show it to you to-night. 



The machine ran at 1500 revolutions per minute, but as both the 

 armature and the field magnets are free to move, their relative rate 

 was 3000 r.p.m. or 50 revolutions per second. The number of pairs of 

 poles is 174, so that frequency is 8700. 



This machine exhibits very well the peculiar difficulty there is in 

 constructing high frequency alternators, for there are no less than 

 K) poles in each inch of the circumference of the discs which form 

 the armature and field respectively. When it is remembered that it is 

 not only necessary to get the 10 poles into 1 inch, but also tlie wires 

 which are between them and the insulation on the wires, it becomes 

 pretty obvious that the constwictional difficulties are very great, 

 especially if the machine is re(iuired to produce large currents, which 

 naturally require large wires to carry them. As an example of an 

 early machine, this machine possesses many points of interest. Firstly, 

 the method adopted of revolving the two parts in opposite directions 

 is extremely interesting, and has been followed in some of the modern 

 machines. It enables twice the frequency to be obtained from the 

 machine with the same risk of bursting the revolving parts as could 

 be obtained if one part were stationary in the ordinary way and the 

 other part revolved. Secondly, the method of winding — namely, the 



