324 Dr. S. L. Hart on some Capillo-electric Effects, 



exterior action of this solenoid, to add to the rotations obtained 

 above43 / '17x2 = 86 / -34. 



In making all the corrections just indicated, it may be said 

 that, for luminous rays traversing a column of carbon bisul- 

 phide contained between the polariscope and the analyzer, and 

 submitted to the electromagnetic action previously defined, 

 corresponding to 24° of deviation in the sine-compass, the 

 plane of polarization of the yellow rays D would undergo by 

 the reversal of the electric current a rotation of 4520 / = 

 75° 20'. 



This number appears exact to about 10'; for the different 

 rotations measured are exact to nearly V; and the only uncer- 

 tainty that could give rise to an error of a few minutes is the 

 error which might have been made regarding the successive 

 positions of the tube in its displacement inside the bobbins. 



When the numbers obtained for the gases are compared 

 with this last number, the error on the rotation of the carbon 

 bisulphide does not amount to - 002. This precision is greater 

 than that which I have been able to realize in the measure- 

 ments relative to the gases. 



[To be continued]. 



XLII. On some Capillo-electric effects, and a Theory of 

 Contact-Potential. By S. Lavington Hart, B.A., D.Sc, 



Scholar of St. Johns College, Cambridge*. 



HAVING occasion, during the course of some experi- 

 ments on contact-potential described further on, to use 

 an electrometer, one electrode of which (A) was in connexion 



Fig. 1. 



with an iron wire dipping in mer- 

 cury contained in a funnel (C), 

 to the neck of which was adapted 

 a fine glass nozzle, while the other 

 electrode (B) was put to an iron 

 cylinder (D) around the falling 

 mercury drops, I observed that 

 there was a deflection of the 

 needle showing A to be positive 

 to B by an amount equal to about 

 *6 D (D being the electromotive 

 force of a sawdust Daniell). This 

 deflection remained the same if 

 the electrode B was put to earth 



* Communicated by the Author, having been read (in Abstract) before 

 Section A of the British Association (September 1881). 



