510 Prof. E. P. Adams and Mr. C. W. Heaps on 



coated for a length of 40 cm. inside and outside with a paste 

 made of powdered graphite and thin shellac. Thick shoulders 

 were left at the ends of the tube for convenience in sup- 

 porting it. For the stretching experiments the tube was 

 supported vertically and weights hung on the lower end. 

 Elongations were read with micrometer-microscopes focussed 

 at two points a fixed distance apart. For the torsion ex- 

 periments the tube was mounted horizontally in a Wertheim 

 torsion apparatus. 



The capacity measurements were made with a Wheat- 

 stone's bridge arrangement supplied with current from a 

 Vreeland sine-wave oscillator *, and with a telephone as 

 detector. Two arms of the bridge were nearly equal wire 

 resistances, wound as nearly capacity- and inductance-free 

 as possible. The steady current value of these resistances 

 were 2566*7 and 2569'7 ohms. The hard rubber condenser 

 'was placed in the third arm of the bridge. In the fourth 

 arm was a variable air-condenser, and parallel with it, a 

 standard guard-ring air-condenser. In order to secure 

 absolute silence in the telephone it was necessary to put a 

 series resistance in the fourth arm. The conditions for 

 silence are then two : 



R 3 R 4 

 Ci Hi Rg , 



47rVCC 1 R 3 R 4 =l; 



where Rj and R 2 are the nearly equal resistances in the first 

 and second arms respectively ; C, the capacity of the con- 

 denser to be measured, R 3 its resistance ; C 1? the whole 

 capacity in the fourth arm, and R 4 the resistance in series in 

 this arm. Since the ratio R 4 /R 3 is very small, the first 

 condition reduces to C/Ci = R 2 /Ri. n is the frequency 

 employed. 



The method of making measurements consisted in balancing 

 as nearly as possible with the variable air- condenser, and 

 making the final adjustment with the standard guard-ring 

 condenser. The condenser was then either stretched or 

 twisted, and the known change in capacity of the standard 

 condenser necessary to secure balance gave the change in 

 capacity sought for. The average results obtained for hard 

 rubber follow : — 



Physical Review, xxvii. p. 286 (1908). 



