210 High Resistances with the D Arsonval Galvanometer, 



the line D. But Professor Dewar lias kindly furnished me 

 with a more recent determination of what he believes to he a 

 purer specimen. The refractive index, at the boiling-point, 

 for the line C is 1-3445, and for the line F is P3528, the 

 specific gravity being 0*55. These figures give 17*53 as the 

 molecular refraction for the line C, and 17*92 for the line F. 

 These will indicate about 17*41 for the line A. The theo- 

 retical value for ethylene, for the line A, is 17*4. 



C 3 . . . . =10*0 

 H^ . . . . = 5-2 



Double-linking . . . =2*2 



17*4 



The coincidence between experiment and theory is very 

 striking. 



XXVI. High Resistances used in connexion with the D'Ar- 

 sonval Galvanometer. By Fkedereck J. Smith, Millard 

 Lecturer in Mechanics, Trinity College, Oxford*. 



THE great utility of the D' Arsonval galvanometer for 

 determining current by the fall of potential method 

 must have been felt by all who have used it for this purpose. 

 When the instrument is thus applied, a large resistance is 

 required in the galvanometer circuit. This is usually made 

 of wire, and costs much. In 1890 I read a short paper before 

 the Ashmolean Society of Oxford on the subject of resistances 

 made of non-metallic substances and mercury jet-contacts. 



Since then resistances of this kind have been constantly in 

 use in the Millard Laboratory, and they have proved them- 

 selves to be both reliable and constant. They are made thus : 

 Dry plaster of Paris and electrotype plumbago are intimately 

 mixed together in suitable proportions!, the composition is then 

 rammed tightly into a glass tube furnished with a platinum 

 terminal ; when nearly full another terminal is introduced 

 and fixed in the substance by compression ; the end of the 

 tube is then closed over the wire in the blowpipe flame, and 

 the resistance is finished. 



In one resistance thus made the glass tube is 10 centim. 

 lono- and 0*4 centim. in internal diameter, and has a resist- 

 ance of about one megohm. Very finely powdered glass 

 mixed with plumbago makes a good composition for high 



* Communicated by the Author. 



t Equal quantities of each in a tube 0*4 centim. diameter and 11*5 

 centim. long gave a resistance of 65,000 ohms. 



