623 EFFECTS OF THE DISCHARGE. 



pressure must be applied to the knob which is in con- 

 tact with the external coating to prevent its gliding off, 

 in which case the discharge might pass through the 

 body of the experimenter. 



During the discharge of the Ley den jar, one electri- 

 city passes through the conductor which connects the 

 two coatings from the inner coating to the outer; the 

 other electricity passes at the same time from the outer 

 coating to the inner, and a recombination of the two 

 electricities takes place within the conductor. This 

 motion of the two electricities, which constitutes the 

 electrical discharge, has also been termed the 4 dis- 

 charge current.' 



The effects of the discharge are of various kinds. 

 Besides the luminous effects in the spark, and those up- 

 on our nervous system in the shock, the discharge pro- 

 duces mechanical effects, such as violent movements, frac- 

 tures, and perforations, and is also a source of heat. Both 

 kinds of effects are considerably heightened if the dis- 

 charge takes place through badly conducting substances. 

 The well-conducting metal parts of the jar and the dis- 

 charger prevent any violent action; they are heated, 

 but so slightly that it escapes observation. But when 

 the discharge takes place through a bad conductor the 

 latter may become so strongly heated as to take fire. 



Among solid bodies, one which is most easily in- 

 flamed by the electric spark, is a mixture of equal por- 

 tions of sulphide of antimony and potassic chlorate, 

 very finely powdered. The mixture is placed in a 

 small apparatus, usually termed ' the electric mortar/ 

 shown in section in fig. 327, made of wood, with a 

 cavity into which two wires project, their ends being 



