2 



Mr. S. Bidwell. 



[Feb. 1,. 



The apparatus consists of a little balance. HK is a light steel rod 

 formed of a fine knitting needle balanced at C on a knife-edge, which 



/ 



.2; 



is fixed at right angles to it. To one end, K, is attached a split tube 

 of thin copper for holding short rods of the different materials to be 

 tested, which are held so as to press at right angles upon another rod, 

 R, supported in a spring clamp. Midway between C and K is hung 

 the scale-pan S, and at H is a sliding counterpoise, which, at the 

 beginning of an experiment, is so adjusted that a weight of *01 grm. 

 in the scale-pan will just bring K and R into contact with each other. 

 Electrical connexion is made through the wires W T and W 2 , the 

 former of which is attached to the clamp holding R, and the latter- 

 communicates with a mercury cup, into which dips a wire attached to 

 the rod HK at the point C. It is evident that the pressure between 

 K and R will be equal to half the weight in the scale-pan S. 



3. Relation between the Pressure of Carbon Contacts and their Resistance. 



Two short rods of electric light carbon, G millims. in diameter, 

 were placed in the instrument in the positions shown at K and R, and 

 the resistance with different pressures measured in the ordinary 

 manner with a Wheatstone's bridge. The means of three series of 

 such measurements made under as nearly as possible the same con- 

 ditions, are given in Table I. 



Table I. — Relation between Pressure and Resistance when measured 

 in the ordinary way. 



Pressure. Eesistance. 

 Grammes. Ohms. 



•25 16-10 



•5 11-00 



