450 RECENT PROGRESS IN PHYSICS, 



Riess has very carefully investigated tlie eifects of gradually in- 

 creasing discharges. To produce very powerful effects he used a 

 battery of 7 jars, with a coating of 2.6 square feet to each jar. 



Long before the quantity of electricity required for ignition had 

 been reached, the wires showed appearances which evinced a forcible 

 penetration of the electricity ; the wire was visibly shaken, small 

 sparks were given off at its ends, particles of its surface were thrown 

 off, rising in the form of a dense vapor. It often happened that the 

 throwing off of larger pieces of glowing metal occurred with the pas- 

 sage of the spark, giving to it a scintillating appearance. Charges 

 still more powerful produced bends in the wire, which appeared ex- 

 actly as though they had been made by an edged tool. We shall give 

 here only one experimental series, showing these phenomena. A 

 platinum wire of 0.0261 line semi-diameter, and 16 lines long, ap- 

 peared as follows : 



AU the phenomena preceding ignition appeared more readily when 

 the wire was not stretched. 



Earlier observers had already noticed a shortening of wires ignited 

 by electrical discharges, which shortening is now explained by the 

 bending mentioned above. 



The sparks spoken of as seen at the ends of the wire depend upon 

 the material of the wire, and upon that of the clamp. The scintil- 

 lating sparks appear in great quantity with iron wire, while with 

 copper none were observed. 



Far more constant than the appearance of sparks is the formation 

 of the vapor which is seen with every metal. The facility with which 

 it is formed, with different metals is the same as for different wires 

 of the same metal. Its formation is promoted by a great number of 

 furrows left by the draw-plate upon the wire ; and Riess has found 

 that it is diminished by carefully polishing the wire. 



§ 52. Laws of electrical ignition. 



1. Ignition in proportion to amount of charge. — A thin platinum 

 wire of 0.116 line diameter, and 26.6 lines length, together with an 

 electrical thermometer containing a platinum wire so thick as to remain 

 uninjured by the strongest discharge, were introduced into the con- 

 ducting circuit. A given number of jars were charged with increasing 

 quantities of electricity until a quantity was attained which produced 



