424 BELL SYSTEM TECHNICAL JOURNAL 



knowledge at this time concerning alternating current theory, a statement 

 in a textbook by Blakesley, with preface dated May, 1889, is illuminating. 

 This author says: 



» 

 "It is often taken for granted that the simple form of Ohm's Law, total E.M.F. 

 ^ total resistance = total current, is true for alternating currents. That is to 

 say, the E.M.F. employed in the formula is taken to be the sum of the impressed 

 E.M.F.'s alone. That there are causes which modifj^ the value of the current as 

 deduced from this simple equation, such as mutual or self-induction, or the action 

 of condensers, is often acknowledged in textbooks, and the values and laws of 

 variation of the current are correctly stated for certain cases of instantaneous 

 contact and breaking of circuit. But the effect of an alternating E.M.F. upon a 

 circuit affected by self-induction, mutual induction, and condensing action, has 

 not been, so far as I know, put into a tangible working form."^^ 



Somewhat similar observations were expressed by Kapp in an article in 

 the Electrician a year or so later. Referring to the paper by Hopkinson 

 mentioned above, he commented as follows: 



". . . he showed that with a certain capacity, periodic time, self-induction and 

 resistance in circuit, the potential difference between the plates of the condenser 

 may be 80 times the E.M.F. of the alternator. Startling as such a result must 

 naturally appear, it failed at the time to attract much attention from practical 

 engineers who, no doubt, preoccupied with the problems relating to continuous- 

 current work, were content to let such an intricate and apparently abstruse prob- 

 lem lie at rest until such time as its consideration should be forced upon them. 

 This time has now come, and what in 1884 was merely an interesting laboratory 

 experiment, having no further application than perhaps the breaking down of a 

 condenser, is at present an interesting practical problem, which the electrical 

 engineer has to face. Phenomena arising from the effects of capacity in alternate- 

 current circuits are forcing those who have to do with such circuits to give attention 

 to the problems connected with the phenomena."'^ 



This quotation gives a fair picture of the situation with respect to electrical 

 engineering around 1890. Familiarity with capacity and resonant effects, 

 it appears, was beginning to grow with the enlargement of professional 

 experience. 



RESON.4NCE IN El.ECTRIC COMMUNICATION 



While resonance, or an approximation thereto, is occasionally encountered 

 in ordinary power engineering and electric lighting, here it is generally a 

 case of something to be avoided, evidence of something gone wrong. An 

 unintentional resonant condition in a power circuit could result in con- 

 siderable damage due to excessive current flow. In electric communication, 

 on the other hand, where frequencies are higher, and where frequency itself 

 is one of the fundamental elements, and currents comparatively small, 

 resonance is of prime importance and may be of great practical value. 



