and Spark-length for Small Values of the latter. 619 



down to a distance of 3fA and a potential of 350 volts. At 

 this point, the curve made a sharp bend and dropped in a 

 straight line to the origin. Earhart also made observations 

 in air at pressures of 228, 152, 40, and 15 cm., and in carbon 

 dioxide at atmospheric pressure. 



This investigation was begun in 1902, (1) in order to 

 discover why Earhart's results did not show the minimum 

 which Peace had observed ; (2) to ascertain whether or not 

 the material of the electrodes affected the position of the 

 tl elbow," as it might be expected to do if the metallic ions 

 took part in the discharge at very small distances ; (3) to 

 extend the range of pressures much lower at these small 

 distances than Earhart had done. Since this investigation 

 was begun, there has appeared a paper by Carr * which 

 covers in an admirable manner the third point mentioned 

 above, but leaves the other two points untouched. 



It would appear from the following results : — 



(1) That the shape of the curve which is obtained with 

 spherical electrodes is indeed precisely what is expected if 

 there is a minimum spark-potential for flat electrodes. 



(2) That the material of which the electrodes are composed 

 exerts an important influence upon the spark-potential at 

 small distances. 



(3) That the carriers of the discharge for small distances 

 come from the metal and not from the gas. 



II. Description of Apparatus. 



The apparatus, which is shown in fig. 2, consisted of an 

 ordinary interferometer to which the two electrodes were 

 attached. To obviate the necessity of using a tangent-screw 

 and still maintain a sufficient accuracy of setting, the main 

 screw of the interferometer was cut with a fifth-millimetre 

 pitch which made it possible to control variations of a tenth 

 of a fringe (i. e. 0*025 /*). 



The movable carriage (C) of the interferometer supported 

 the plane electrode (EJ, a disk of brass about 1 inch in 

 diameter screwed tightly to a taper shaft which was set in a 

 carefully-ground socket in the carriage and held in place by 

 a spring (S). A thin gear (Gr 4 ) 1^ inch in diameter was 

 also concentric on the same shaft and, when engaged in the 

 gear (Gr 3 ) above it which was operated outside of the box, 

 could be turned on its axis. In order to get rid of the 

 troublesome backlash in the nut (N), to which the sliding- 

 carriage was fastened, a stiff coiled spring (W) running 



* Carr, Proc. Roy. Soc. lxxi. p. 374. 

 2 U2 



