650 Mr. Nikola Tesla [Feb. 4, 



but at excessively bigli degrees of exhaustion there seems to be, 

 beyond a certain and rather small size of the vessel, no perceptible 

 difference in the heating. 



The shape of the vessel is also of some importance, and it has 

 been found of advantage for reasons of economy to employ a spheri- 

 cal bulb with the electrode mounted in its centre, where the re- 

 bounding molecules collide. 



It is desirable on account of economy that all the energy supplied 

 to the bulb from the source should reach without loss the body to 

 be heated. The loss in conveying the energy from the source to 

 the body may be reduced by employing thin wires heavily coated 

 with insulation, and by the use of electrostatic screens. It is to be 

 remarked, that the screen cannot be connected to the ground aa under 

 ordinary conditions. 



In the bulb itself a large portion of the energy supplied may be 

 lost by molecular bombardment against the wire connecting the body 

 to be heated with the source. Considerable improvement was effected 

 by covering the glass stem containing the wire with a closely fitting 

 conducting tube. This tube is made to project a little above the glass, 

 and prevents the cracking of the latter near the heated body. The 

 effectiveness of the conducting tube is limited to very high degrees 

 of exhaustion. It diminishes the energy lost in bombardment for 

 two reasons : firstly, the charge given up by the atoms spreads over 

 a greater area, and hence the electric density at any point is small, 

 and the atoms are repelled with less energy than if they would strike 

 against a good insulator ; secondly, as the tube is electrified by the 

 atoms which first come in contact with it, the progress of the following 

 atoms against the tube is more or less checked by the repulsion which 

 the electrified tube must exert upon the similarly electrified atoms. 

 This, it is thought, explains why the discharge through a bulb is 

 established with much greater facility when an insulator than when 

 a conductor is present. 



During the investigations a great many bulbs of different construc- 

 tion, with electrodes of different material, were experimented upon, 

 and a number of observations of interest were made. 



It was found that the deterioration of the electrode is the less the 

 higher the frequency. This was to be expected, as then the heating 

 is effected by many small impacts, instead of by fewer and more violent 

 ones, which shatter quickly the structure. The deterioration is also 

 smaller when the vibration is harmonic. Thus an electrode, main- 

 tained at a certain degree of heat, lasts much longer with currents 

 obtained from an alternator, than with those obtained by means of 

 a disruptive discharge. One of the most durable electrodes was 

 obtained from strongly compressed carborundum, which is a kind of 

 carbon recently produced by Mr. E. G. Acheson. From experience, 

 it is inferred, that to be most durable, the electrode should be in the 

 form of a sphere with a highly polished surface. 



In some bulbs refractory bodies were mounted in a carbon cup 



