62 EEPORT — 1876. 



was led to try a third experiment in verification of Ohm's law. If there be 

 a periodic variation of the primary of an induction-coil, the time integral of 

 the electromotive force in the secondary through one complete oscillation 

 will be zero ; but if the variation consist of a sharp break, although this 

 law holds, yet the oscillation in the secondary may be divided into two parts, 

 in one of which the maximum intensity is very much greater than in the 

 other. If it be true, then, that a more intense current encounters less 

 resistance than a less intense current, clearly the law above stated can no 

 longer hold ; the law has, in fact, been deduced on the supposition that the 

 resistance is independent of the strength of the current. 



It follows, therefoi'e, that if we send the induction- currents from the 

 secondary of an induction-coil, whose primary is made and broken by a tuning- 

 fork, througli a helix of fine wire to make sure of bringing out the efi'ects we 

 are looking for, then the needle of a galvanometer introduced into the 

 secondary will bo deflected so as to indicate a current in the direction of the 

 current due to breaking the primary. 



Certain anomalous, and at first sight contradictory, results led the writer 

 to study the behaviour of a galvanometer under these circumstances. The 

 result was the suggestion of a theory which explained the anomalies com- 

 pletely, and indicated the existence of certain other phenomena which were 

 afterwards observed. 



The results are, so far as the writer has been able to learn, partly new. 

 Although not of suificicni importance in connexion with the present subject 

 to require detailed mention here, yet it was thought best to state the results 

 so far as they bear on the question, reserving a detailed account for publica- 

 tion elsewhere *. 



It was found that, under the circumstances indicated above, the indication 

 of a galvanometer is a function of the ratio of the strengths of the magnetic 

 field when there is no current and when the currents are passing, and also 

 of the position of equilibrium of the needle when there is no current. 



Theorj!- and observation give alike, among others, the following peculiari- 

 ties : — 



1. If the ratio of the magnetic forces due to the currents to that acting 

 on the needle when there is no current does not exceed a certain quantity, 

 then if the position of rest of axis of the needle is inclined at an angle a ( < 90°) 

 to the plane of the coil-windings, the effect of the alternating currents is to 

 increase that angle, so that, according as the needle is deflected one way or 

 the other by means of the deflecting magnet, we get opposite eflfects. 



The eff'ect is zero when n is zero. 



2. If the above-mentioned ratio exceeds a certain value, the position of the 

 needle parallel to the windings (i. e. for a = 0) becomes unstable, and there now 

 appear two positions of equilibrium of equal inclination cither way to the 

 coU-windings. Either of these the needle will take up and keep if brought 

 there with sufficiently small velocit}^ 



The greater the ratio, the more nearly these two positions approach to 

 parallelism with the plane of the coil-windings. 



The last-mentioned phenomenon was described long ago by Poggendorfi", 

 under the name of ' ' doppelsinnige Ablenkung," and was and has been regarded 

 apparently as an unstable phenomenon. 



The first-mentioned form of the phenomenon has not, so far as the writer 

 knows, been hitherto described anywhere. 



In repeating Dr. Schuster's experiments by superposing a small current 

 * PliU. Mag. [v.] vol. ii. p. 401. 



