134 On a new Insulating Support 



for which, from (39), we may also write 



. . . . (46) 



pb + q — abp* 



The value of ^expresses the number of what are called the dead 

 turns. 



I reserve for a future paper further applications of the chief 

 equations developed in this paper, more especially to the trans- 

 mission of force by dynamo-electrical machines. 



XVI. On a new Insulating Support. 



By Professor Silvanus P. Thompson, B.A., D.Sc* 



[Plate III.] 



INSULATING supports consisting of rods rising through 

 the necks of glass jars containing concentrated sulphuric 

 acid, for the purpose of absorbing moisture which otherwise 

 would condense upon the glass, appear to have been first intro- 

 duced in practice by Sir William Thomsonf. Similar devices 

 have been more recently designed by Mascart^:, by Professor 

 Clifton §, and by Professors Ayrton and Perry. The apparatus 

 of Mascart differs only from the original design of Sir W. 

 Thomson in having the central support of glass solidly fused 

 to the bottom of the jar which holds the acid, and in having 

 the jar formed with a narrow neck instead of a wide one. This 

 construction, though convenient in point of portability and 

 solidity, renders necessary the addition of a tubulure at the 

 side of the vessel by which to introduce the acid. The cost of 

 the apparatus, which cannot be made except by a professional 

 glass-blower, is consequently considerable. 



Such supports are, in spite of their cost, of so great utility, 

 especially in a humid climate, that a cheaper substitute of 

 equally high insulation is a desideratum. In the electrical 

 laboratory of University College, Bristol, insulating supports 

 of the type about to be described are found of very great use. 



A piece of combustion-tube, about 20 centim. long and 1*4 

 centim. diameter, is fused together at one end, and the closed 

 end is slightly enlarged by blowing, and then slightly flattened 

 at its extremity. This tube, which forms the central support, 



* Communicated by the Physical Society ; read December 8, 1883. 



t Proc. Hoy. Soc. June 1867, and i Reprint of Papers on Electrostatics 

 and Magnetism/ p. 322. See also the figure given on page 14 of Max- 

 well's i Elementary Treatise on Electricity.' 



\ Journal de Physique, t. vii. p. 217 (1878) ; < Nature,' xviii. p. 44 (1878) ; 

 see also Wiedemann's Electrieitat, Bd. i. p. 16. 



§ Proc. Eoy. Soc. No. 182, p. 300 (1877). 



