﻿544 Royal Society : — 



the Physical Laboratory of the University of Glasgow." By John 

 C. Gibson, M.A., and Thomas Barclay, M.A. 



This paper describes the instruments and processes employed in a 

 series of experiments on the specific inductive capacity of paraffine, 

 and the effect upon it of variations of temperature. The instruments 

 described are the platymeter and the sliding condenser. The former 

 of these was, in a rudimentary form, shown to the Mathematical and 

 Physical Section of the British Association at its Glasgow Meeting in 

 1855, by W. Thomson. It consists of two equal and similar conden- 

 sers employed for the comparison of electrostatic capacities. The 

 sliding condenser is a condenser the capacity of which may be varied 

 by known quantities by altering the effective area of the opposed 

 surfaces. By means of these two instruments, along with the qua- 

 drant electrometer, the capacity of a condenser may be determined 

 by equalizing the sliding condenser to it. The method of working, 

 and the electrical actions upon which it depends, are described in 

 detail. In order to determine the capacity of the sliding condenser 

 at the lower extremity of its range, a spherical condenser, so con- 

 structed that its capacity could be accurately determined in absolute 

 measure, was employed. An apparent discrepancy in the results 

 obtained, arising from an inequality in the condensers forming the 

 platymeter, is then considered, and the method of deducing the true 

 result investigated. A series of experiments are then described which 

 gave 1*9/5 as the specific inductive capacity of paraffine, that of air 

 being taken as unity, but failed to show whether this alters with va- 

 riations of temperature. An improved form of condenser, composed 

 of concentric brass cylinders with paraffine for the dielectric, and the 

 results obtained from it, are then described. The measurements 

 made at different temperatures show no variation of specific inductive 

 capacity. In order to allow to the paraffine freedom of expansion 

 with temperature, another form of condenser was employed, and the 

 same results obtained. A series of experiments were then made on 

 the expansion of paraffine with temperature, in order to estimate the 

 effect of this upon the capacity of paraffine condensers. As a mean 

 of the results, it was found that the linear expansion of paraffine at 

 9° C. is '000237 per degree. Some further measurements of the 

 cylindrical condenser were made with the same result as before. 

 Thus all the measurements of this condenser made at temperatures 

 ranging from — 1 2°* 15 to 24°'35 C. show no variation of specific in- 

 ductive capacity of paraffine with temperature. This was found to 

 be 1*977, that of air being taken as unity. 



In a note added to the paper a description is given of an improved 

 form of sliding condenser. 



Feb. 16. — General Sir Edward Sabine, K.C.B., President, in the 



Chair. 

 The following communication was read : — 



" On a Registering Spectroscope." By Willian Huggins, LL.D., 

 D.C.L., F.R.S. 



The short duration of the totality of the solar eclipse of December 



