LESSORS IN ELECTRICITY. 



8*3 



preference to crossing the space s ; but 

 the returning stroke is too strong and 

 sudden to find a sufficiently open channel 

 through the table and chain, and on the 

 discharge of the prime conductor the 

 spark is seen. 



It was tho action of the return shock 

 upon a dead frog's limbs, observed in the 

 laboratory of Professor Galvani, that led 

 to Galvani's experiments on animal elec- 

 tricity ; and led further to the discovery, 

 by Volta, of the electricity which bears 

 his name. 



32. The Leyden Battery, its Currents, 

 and some of their Effects. 



In the ordinary Leyden battery de- 

 scribed in 19 all the inner coatings are 

 connected together, and all the outer 

 coatisgs arc also connected together. 

 Such a battery acts as a single large jar 

 of extraordinary dimensions. 



Wires are warmed by a moderate elec- 

 tric discharge ; by augmenting the charge 

 they arc caused to glow ; with a strength- 

 ened charge the metal is torn to pieces ; 

 fusion follows ; and by still stronger 

 charges the wires are reduced to metallic 

 dust and vapor. 



For such experiments the wire must be 

 thin. Without resistance we can have 

 no heat, and when the wire is thick we 

 have little resistance. The mechanism 

 of the discharge, as shown by the figures 

 produced, is different in different wires. 

 The figure produced by the dust of a def- 

 lagrated silver wire on white paper is 

 shown in fig. 58. 



When the discharge of a powerful bat- 

 tery is sent through a long steel chain 

 with the ends of its links unsoldered, the 

 sparks between the unsoldered links carry 

 the incandescent particles of the steel 

 along with them. These are consumed 

 in the air, a momentary blaze occurring 

 along the entire chain. Chain cables 

 have been fused by being made the chan- 

 nels of a flash of lightning. 



Retaining our conception of an electric 

 fluid, at this point we naturally add to it 

 the conception of a current. It is the 

 electric current which produces the effects 

 just described. In many of our former 

 experiments we had electricity at rest 

 (static electricity), here we have electric- 

 ity in motion (dynamic electricity). 

 Sending the current from a battery 



through a fiat spiral (the primary) formed 

 of fifty or sixty feet of copper wire, and 

 placing within a little distance of it a 

 second similar spiral (the secondary) with 

 its ends connected ; the passage of tho 

 current in the first spiral excites in tho 

 second a current, \vhich is competent to 

 deflagrate wires, and to produce all the 

 other 5 effects of the electrical discharge. 

 Even when the spirals are some feet asun- 

 der, the shock produced by the second- 

 ary current is still manifest. 



The current from tbe secondary spiral 

 may be carried lound a third ; and this 

 third spiral may be allowed to act upon 

 u fourth, exactly as the primary did upon 

 the secondary. A tertiary current is thus 

 evoked by the secondary in. the fourth 

 spiral. 



Carrying this tcitiary current round a 

 fifth spiral, and causiig it to act induc- 

 tively upon a sixth, we obtain in the lat- 

 ter a current of the fourth order. In this 

 way we generate a long progeny of cur- 

 rents, all of them having the current sent 

 from the battery through the first spiraJ 

 for a common progenitor. To Prof. 

 Ilenry of the United States, and to Prof. 

 Riess of Berlin, we are indebted for the 

 investigation of the laws of these cur- 

 rents. These researches, bow eve*,, were 

 subsequent to, and were indeed suggest- 

 ed by, experiments of a similar character 

 previously made by Faraday with Voltaic 

 electricity. 



Besides the electricity of friction and 

 induction we have the following sources 

 and forms of this power. 



The contact of dissimilar metals pro- 

 duces electricity. 



The contact of metals with liquids pro- 

 duces electricity. 



A mere variation of the character of the 

 contact of two bodies produces electricity. 



Chemical action produces a continuous 

 flow of electricity (Voltaic electricity). 



Heat, suitably applied to dissimilar 

 metals, produces a continuous flow of 

 electricity (thermo-electricity). 



The heating and cooling of certain 

 crystals produce electricity (pyro-electric- 

 ity). 



The motion of magnets, and of bodies 

 carrying electric currents, produces elec- 

 tricity (magneto-electricity). 



The friction of sand against a metal 



