222 THE POPULAR SCIENCE MONTHLY. 



The radiometer of Crookes imported a new feature into these inquiries, 

 which at the present time occupy the attention of leading physicists in 

 all countries. 



The means usually employed to produce electrical discharge in 

 vacuum-tubes was Rukrnkorff's coil ; but Mr. Gassiot first succeeded 

 in obtaining the phenomena by means of a galvanic battery of 3,000 

 Leclanche cells. Dr. De La Rue, in conjunction with his friend Dr. 

 Hugo Miiller, has gone far beyond his predecessors in the production 

 of batteries of high potential. At his lecture " On the Phenomena of 

 Electric Discharge," delivered at the Royal Institution, in January, 

 1881, he employed a battery of his invention consisting of 14,400 cells 

 (14,832 Volts), which gave a current of 0'054 Ampere, and produced 

 a discharge at a distance of 0*71 inch between the terminals. During 

 last year, he increased the number of cells to 15,000 (15,450 Volts), 

 and increased the current to - 4 Ampere, or eight times that of the 

 battery he used at the Royal Institution. 



With the enormous potential and perfectly steady current at his 

 disposal, Mr. De La Rue has been able to contribute many interesting 

 facts to the science of electricity. He has shown, for example, that 

 the beautiful phenomena of the stratified discharge in exhausted tubes 

 are but a modification and a magnification of those of the electric arc 

 at ordinary atmospheric pressure. Photography was used in his ex- 

 periments to record the appearance of the discharge, so as to give a 

 degree of precision otherwise unattainable in the comparison of the 

 phenomena. He has shown that between two points the length of the 

 spark, provided the insulation of the battery is efficacious, is as the 

 square of the number of cells employed. Mr. De La Rue's experi- 

 ments have proved that at all pressures the discharge in gases is not 

 a current in the ordinary acceptation of the term, but is of the nature 

 of a disruptive discharge. Even in an apparently ])erfectly steady dis- 

 charge in a vacuum-tube, when the strata as seen in a rapidly revolv- 

 ing mirror are immovable, he has shown that the discharge is a pulsat- 

 ing one ; but, of course, the period must be of a very high order. 



At the Royal Institution, on the occasion of his lecture, Mr. De La 

 Rue produced, in a very large vacuum-tube, an imitation of the aurora 

 borealis ; and he has deduced from his experiments that the greatest 

 brilliancy of aurora displays must be at an altitude of from thirty- 

 seven to thirty-eight miles a conclusion of the highest interest, and 

 in opposition to the extravagant estimate of 281 miles at which it had 

 been previously put. 



The President of the Royal Society has made the phenomena of 

 electrical discharge his study for several years, and resorted in his im- 

 portant experiments to a special source of electric power. In a note 

 addressed to me, Dr. Spottiswoode describes the nature of his inves- 

 tigations much more clearly than I could venture to give them. He 

 says : " It had long been my opinion that the dissymmetry shown in 



