CHAP, iv.] ELECTRICITY AND MAGNETISM. 241 



since 42 x io 6 ergs = i gramme- water-degree of heat), 

 and Q and V are expressed in C. G. S. units. 



When a powerful discharge takes place through very 

 thin wires, they may be heated to redness, and even 

 fused by the heat evolved. Van Marum thus once 

 heated 70 feet of wire by a powerful discharge. A 

 narrow strip of tinfoil is readily fused by the charge of 

 a large Leyden jar, or battery of jars. A piece of gold 

 leaf is in like manner volatilised under the sudden heat- 

 ing of a powerful discharge ; and Franklin utilised this 

 property for a rude process of multiplying portraits or 

 other patterns, which, being first cut out in card, were 

 reproduced in a silhouette of metallic particles on a 

 second card, by the device of laying above them a film 

 of gold or silver leaf covered again with a piece of card 

 or paper, and then transmitting the charge of a Leyden 

 battery through the leaf between the knobs of a universal 

 discharger. 



289. Luminous Effects. The luminous effects 

 of the discharge exhibit many beautiful and interesting 

 variations under different conditions. The spark of the 

 disruptive discharge is usually a thin brilliant streak of 

 light. When it takes place between two metallic balls, 

 separated only by a short interval, it usually appears 

 as a single thin and brilliant line. If, however, the 

 distance be as much as a few centimetres, the spark 

 takes an irregular zig-zag form. In any case its path is 

 along the line of least resistance, the presence of minute 

 motes of dust floating in the air being quite sufficient to 

 determine the zig-zag character. In many cases the 

 spark exhibits curious ramifications and forkings, of 

 which an illustration is given in Fig. 107, which is drawn 

 of one eighth of the actual size of the spark obtained 

 from a Cuthbertson's electrical machine. The discharge 

 from a Leyden jar affords a much brighter, shorter, 

 noisier spark than the spark drawn direct from the 

 collector of a machine. The length (see Art. 291) 

 R 



