RECENT PROGRESS IN PHYSICS. 361 



dentally in the jet, had with the other taken hold of the lever to adjust 

 the weight of the safety valve, when a spark passed between his hand 

 and the lever, and he received a severe electrical shock. 



Armstrong went to the place, and verified this statement *, hut the 

 sparks were not so powerful as they had been before, whicli he as- 

 cribed to the circumstance that, the day before his arrival, the boiler 

 had been cleaned by the removal of a thin calcareous incrustation ; 

 this, however, his subsequent investigations showed had no influence 

 whatever upon the excitation of electricity. 



In continuing his investigations, Armstrong stood upon an insula- 

 ting stool, and found that then the sparks were much stronger. A 

 metallic rod, with a brass plate at one -end and a ball at the other, 

 was held by an insulating handle, with the plate in the jet of steam ; 

 the whole insulated conductor showed signs of electricity, and sparks 

 could be drawn from the knob. When the knob was brought within 

 a quarter of an inch of the boiler, between sixty and seventy sparks 

 passed in a minute. The greatest distance between the knob and the 

 boiler at which a spark appeared was one inch. 



The load upon the valve was thirty-five pounds to the square inch. 

 The electrical excitement increased and decreased with the tension of 

 the steam in the boiler. 



The electricity of the steam and of the conductor held in it was 

 positive. 



Investigations made upon other boilers gave similar results ; very 

 powerful sparks were obtained from a locomotive. Armstrong stood 

 upon an insulating stool, and took in one hand a light iron rod, which 

 he held in the steam escaping from the safety valve. When the other 

 hand was brought near an uninsulated conductor, he obtained sparks 

 an inch long. "The length of the sparks increased to two inches when 

 tlie rod was held five or six feet above the safety valve. 



Even from the cloud of steam in the engine house iii which the 

 locomotive stood, electricity could be drawn as by a lightning-rod 

 from a storm cloud. 



When the upper end of the rod held in the hand was provided with 

 a brush of wire, sparks four inches long could be drawn from a knob 

 on the lower end. 



To discover the negative electricity corresponding to the positive of 

 the escaping steam, the locomotive was raised from the rails, and its 

 wheels placed upon insulating supports. Each of these supports con- 

 sisted of three blocks of dried wood, covered with pitch, and separated 

 by layers of pitch and paper. To avoid increasing the height, and 

 at the same time to extend the insulating surface, the middle block 

 was made much wider than the others. The water in the boiler was 

 then made to boil. As long as the steam was confined, the boiler 

 gave no signs of electricity ; but as soon as it was allowed to escape, 

 the boiler became strongly negative. 



The sparks from the boiler were never more than one inch long, 

 and this is easily understood when we consider that the electricity of 

 the boiler, on account of the numerous angles and prominences of the 

 locomotive, could not attain a high tension. 



The experiments which Armstrong made to discover the source of 



