iiisroRV or electrical resoxaxce 421 



llu' inductance and capacity, all in their proper units). KlTectin^' this 

 condition would of course be a case of "electrical resonance", brought about 

 by electrical "tuning", though Maxwell himself did not spccil'icaily make 

 use of these terms. 



It is apparent, therefore, that all the knowledge necessary for the realiza- 

 tion of electrical tuning was available in 1868 when this little communication 

 of Maxwell's was published. Yet it was some years before electrical reson- 

 ance was encountered to any extent in practice, or any practical use deliber- 

 ately made of it. It was something known about by advanced physicists, 

 |)erhaps, but outside the ken of most of those experimenting with electricity 

 at that period; it must be remembered that there was no profession of 

 electrical engineering as early as this. Not until 1884 do we find any 

 other published discussion of the effect of a condenser in an alternating 

 current circuit. This was in a technical paper by John Hopkinson, Cam- 

 bridge trained physicist and later to become one of the foremost electrical 

 engineers of his day. The paper had to do with alternating current theory 

 and the operation of alternating current machines, and in it occurs the 

 following : 



"Some time ago Dr. Muirhcad told mc that the effect of an alternating-current 

 machine could be increased by connecting it to a condenser. This is not ditlicult 

 to explain: it is a case of resonance analogous to those which are so familiar in the 

 theory of sound and in many other branches of physics. 



"Take the simplest case, though some others are as easy to treat. Imagine an 

 alternating-current machine with its terminals connected to a condenser; it is 

 required to find the amplitude of oscillation of jjotcntial between the two sides of 

 the condenser. . ."'" 



By setting up an equation similar to that used by Maxwell the required 

 expression was found; and by assuming certain reasonable values of the 

 frequency, resistance, inductance and capacity, he calculated that the 

 amplitude of the potential difference across the condenser might be many 

 times the voltage of the generator. It is apparent from the language he 

 used that he had a perfectly clear understanding of electrical resonance. 



The First Electrical Resonance Curve 



Following Maxwell, there was another brilliant young physicist destined 

 to become famous who showed a thorough acquaintance with electrical 

 resonance and who made good use of it in his celebrated researches. This 

 was Heinrich Hertz, in Germany, who applied it in the detection of electric 

 waves produced by a spark discharge, the oscillatory nature of which had 

 already been well investigated, as we have seen. The simple device he 

 used for exploring the field in the vicinity of the discharge was a rectangle 

 or circle of wire containing a minute spark gap, the loop being of such 



