n I STORY OF ELECTRICAL RESONANCE 419 



roLirst" of a discussion at a iiieetin<i; of the A.l.K.K. Some preliminaries to 

 tliis story may first be related. 



For long the RuhmkorlT induction coil and the magneto-electric machine 

 had been familiar objects in physical laboratories. In 1866 there appeared a 

 description of Henry Wilde's striking experiments in which he virtually 

 reinvented and introduced the separately excited dynamo, passing the small 

 (commutated) lurrent from a magneto-electric generator through the field 

 magnet windings of another machine, from the armature of which a very 

 much larger current was obtained. Sir William (irove, reading of these 

 experiments, had the idea that a magneto might also be used advantageously 

 to operate a Ruhmkorfif coil with an alternating current. Induction coils 

 had always been excited by means of a battery with a self-acting circuit 

 breaker to interrupt the primary current, and in order to prevent sparking 

 at the co^ttacts and to stop the current more abruptly a condenser was 

 connected across the contact terminals. Grove screwed up and kept closed 

 the contact breaker, thus short-circuiting the condenser, and applied an 

 ordinary medical magneto-electric machine to the primary terminals of his 

 induction coil. To his surprise he found that he could get no secondary 

 discharge at all; but by holding open the contact breaker, and so putting the 

 condenser permanently in series with the primary coil and the armature 

 of the magneto-electric machine, he obtained sparks nearly a third of an 

 inch in length between the ends of the secondary. He saw that the effect 

 was dependent upon the presence of the condenser in the circuit; "But why 

 there should be no effect, or an appreciable one, when the primary circuit 

 is completed, the current being alternated by the rotation of the coils of the 

 magneto-electric machine, I cannot satisfactorily explain," he said.^ 



And now Professor Pupin's story: 



". . . Maxwell, I think, was the first to show the el^ect of introducing a condenser 

 into an alternating current circuit, and it is very interesting to observe this circum- 

 stance. Maxwell was spending an evening with Sir William Grove who was then 

 engaged in experiments on vacuum tube discharges. He used an induction coil 

 for this purpose, and found that if he put a condenser in parallel [it was in series, 

 rather] with the primary circuit of his induction coil he could get very much larger 

 sparks, which meant, of course, that he got a verj- much larger current through 

 his primary coil, an alternating current generator being used to feed the primary. 

 He could not see why. Maxwell, at that time, was a young man. That was about 

 1865, if I do not err. [It was 1868.] Grove knew that Maxwell was a splendid 

 mathematician, and that he also had mastered the science of electricity as very 

 few men had, especially the theoretical part of it, and so he thought he would ask 

 this young man how it was possible to obtain such powerful currents in the primary 

 circuit by adding a condenser. Maxwell, who had not had very much experience 

 in experimental electricity at that time, was at a loss. But he spent that night in 

 working over his problem, and the next morning he wrote a letter to Sir William 

 Grove explaining the whole theory of the condenser in multiple [series] connection 



