226 Mr. T. Gray on the Electric Resistance 



events as an ordinary tiling. It may come in useful occasion- 

 ally when things are wanted in a hurry. 



Emery Grinder for Tools. — This is an excellent machine of 

 American make, small, cheap, and very efficient. It is screwed 

 down upon the filing-bench, where it occupies hardly any 

 space, and is driven from the shafting above. The tools ground 

 by this machine are quite a different thing from tools ground 

 on the ordinary grindstone, and they take a small fraction of 

 the usual time to grind. A few minutes before or after the 

 regular work of the engine suffice to keep the tools in such 

 order as could hardly be attained otherwise by an occasional 

 half day at the grindstone. 



The carpenter's bench is a common one. The filing-bench 

 is of great solidity, let into the wall and floor. It supports two 

 vices and the emery grinder. 



I will not now enter into any details as to the acoustic 

 apparatus, but leave that for a subsequent occasion. 



St. John's College, Oxford, 

 July 29, 1880. 



XXXII. On the Electric Resistance of Glass at different Tem- 

 peratures. By Thomas Geay, B.Sc, C.E., Demonstrator 

 in Physics and Instructor in Telegraphy , Imperial College of 

 Engineering j Tokio, Japan*. 



IN the following pajoer are described the results of some 

 experiments on the electric resistance of glass at different 

 temperatures. These experiments were undertaken with the 

 view of finding whether the variation of resistance with tem- 

 perature followed a law sufficiently definite to allow the resist- 

 ance at low temperatures, of highly insulating glass, to be 

 approximately calculated from its measured resistance at high 

 temperatures. My chief object was to use this law, with results 

 obtained by the galvanometric method of measuring resistance, 

 for the comparison of specific resistances of different kinds of 

 glass. 



Published results of experiments on various substances, such 

 as gutta-percha, india-rubber, glass, &c, as well as some expe- 

 riments of my own on other substances, led me to expect that 

 the resistance of glass might possibly be expressed by an 

 equation of the form 



logK=C-C% 



where R is the resistance of the material, t the temperature, 

 and C and C constants depending on the nature of the ruate- 



* Communicated by the Author. 



