622 Mr. G. Moody Hobbs on the Relation between P. I). 



discharge was to take place (about 0*01 volt)*. When contact 

 had been established, the reading o£ the reference point was 

 taken, and the surfaces drawn apart until the required number 

 of fringes had passed the fiducial point. The main discharge 

 circuit was then closed through the switches K 3 and K 4 and 

 the plunger in the water resistance (K 2 ) raised very slowly 

 until the potential of discharge was reached. In the neigh- 

 bourhood of the discharge potential the P.D. was raised a 

 volt at a time with a wait of a minute between in order that 

 the effect of lag, first noticed by Warburg t? might be 

 eliminated. The discharge was always indicated by the 

 dropping of the pointer of the voltmeter. The electrodes 

 were then drawn apart and fresh surfaces presented for 

 contact by turning Gr 3 and H, and after a wait of 8 or 10 

 minutes to allow the gas to resume its normal condition 

 another discharge was taken. The same pressure was 

 maintained throughout each series of observations. 



III. Results in Air with Brass Electrodes. 



(1) Comparison of Curve for Atmospheric Pressure with 

 Earliartfs Curve. — The results with brass electrodes at various 

 pressures are given in Table I., and represented graphically in 

 fig. 4 (PL XIII.). In order to make comparisons the more 

 easily, the curve for atmospheric pressure has been plotted with 

 that of Earhart, in fig. 1, the latter being the dotted one. It will 

 be seen that the discrepancy which exists at the " elbow "' of the 

 curve is really only apparent; for Earhart's results (indicated 

 by X ) show a discharge-potential which is practically stationary 

 at 348 volts for distances from 5\to 13*5X. The difference 

 in slope from the elbow to the origin in the two curves is shown 

 by fig. 13 to be due to the metal of which the electrodes were 

 composed, Ear hart's surfaces being nickel. The agreement 

 in the case of the curves for 40 and 15 cm. pressure is not so 

 good, the errors in Earhart's observations being due probably 

 to imperfect drying of the air. 



* Note. — After the departure of the manuscript of this article, the 

 attention of the writer was called to the fact that the indicating potential 

 for different metals should not be below certain minimum values. This 

 explained a difficulty experienced in obtaining definite contact between 

 certain metals, a fact which is mentioned later in the article to explain 

 rather scattered results for zinc, antimony, &c. These curves have been 

 repeated, using a higher indicating potential, and much more consistent 

 results have been obtained. The new results, together with observations 

 on several metals not previously used, will be published in a subsequent 

 article. 



f Warburg, Ann. d. Phyf. vol. lxii. p. S85. 



