240 ELEMENTARY LESSONS ON * [CHAP. iv. 



rubbing of a person's shoes against the carpet, as he 

 shuffles across the floor, generates sufficient electricity to 

 enable sparks to be drawn from his body, and he may 

 light the gas by a single spark from his outstretched 

 finger. Gunpowder can be fired by the discharge of a 

 Leyden jar, but the spark should be retarded by being 

 passed through a wet thread, otherwise the powder will 

 simply be scattered by the spark. 



The Electric Air- Thermometer, invented by Kin- 

 nersley, 1 serves to investigate the heating powers of the 

 discharge. It consists of a glass vessel enclosing air, 

 and communicating with a tube partly filled with water 

 or other liquid, in order to observe changes of volume or 

 of pressure. Into this vessel are led two metal rods, 

 between which is suspended a thin wire, or a filament 

 of gilt paper ; or a spark can be allowed simply to cross 

 between them. When the discharge passes the enclosed 

 air is heated, expands, and causes a movement of the 

 indicating column of liquid. Mascart has further de- 

 veloped the instrument by making it self-registering. 

 The results of observation with these instruments are 

 as follows : The heating effect produced by a given 

 charge in a wire of given length is inversely proportional 

 to the square of the area of the cross section of the wire. 

 The heating effect is greater, the slower the discharge. 

 The total heat evolved is jointly proportional to the 

 charge, and to the potential through which it falls. In 

 fact, if the entire energy of the discharge is expended 

 in producing heat, and in doing no other kind of work, 

 then the heat developed will be the thermal equivalent 



of \ QV, or will be -y units of heat, where J repre- 

 sents the mechanical equivalent of heat, (J 42 million ; 



1 This instrument differs in no essential respect from that devised ninety 

 years later by Riess, to whom the instrument is often accredited. Riess, 

 however, deduced quantitative laws, while Kinnersley contented him- 

 self with qualitative observations. Snow Harris also anticipated Riess in 

 several points of his researches. 



