394 Mr. E. C. Rimington on an Air-Core Transformer 



a quick downward jump of the fringes at or immediately after 

 the instant of the spark. Under good optical conditions, and 

 at potentials high and low, the effect was perfectly regular, 

 and was as distinct and pure as that in oil of colza, though 

 apparently not quite so large. 



(2) Plane of polarization of the pencil BF horizontal : 

 rise of fringes indicates relative retardation of BF. The 

 disturbance of the fringes was greatly reduced as the experi- 

 ment went on, till at hist there was nothing left hut a set of 

 slow movements, very irregular and very small, sometimes 

 invisible. In the midst of these as in their absence, and in a 

 long set of observations taken at different potentials from low 

 to highest, there was no trace ever seen of a jump of the 

 fringes at the instant of the spark. It appears, therefore, that 

 in this negative dielectric, as in oil of colza, the total optical 

 effect of electric strain is an acceleration of the vibration 

 which is directed along the line of force. 



The conclusion to be drawn from the preceding experiments 

 has been stated already by anticipation ; but I repeat it finally 

 in other terms as follows : — 



If light pass through an electrostatically strained medium at 

 right angles to the lines of force, and he represented by two com- 

 ponent lights whose planes of polarisation are respectively 

 parallel to the lines of force and perpendicular, then the proper 

 and immediate optical effect of the electric strain is a change of 

 velocity of the latter component *. 



The use of the words proper and immediate in this state- 

 ment may be thought objectionable ; but some such words are 

 required for the purpose here chiefly intended, which is, to 

 exclude those undoubtedly remote effects of electric action 

 that appeared as disturbances in all the experiments. 



Glasgow, February 26, 1894. 



XXXV. On the Behaviour of an Air- Core Transformer token 

 the Frequency is below a certain Critical Value. By E. 0. 



Rimington j. 



[Plates X., XI.] 



IT is usually supposed in the case of a transformer whose 

 primary is connected to terminals having an alternating 

 potential difference of constant value between them, that the 



* The change of velocity in the case of any positive dielectric is of 

 course a decrease. 



f Communicated by the Physical Society : read October 27, 1893. 



