204 



bundle of iron wire, and furnished with a mechanical contri- 

 vance for rapidly breaking and restoring contact. In this 

 primary coil machine the same phenomena are presented as 

 appeared with the long coil of wire, — when contact with the 

 battery is perfect the current passes through it from the zinc 

 to the copper, silver, or platinum plate ; at the instant of con- 

 tact, however, a flash in the opposite direction passes through 

 it, and, when broken, an induced current rushes along the 

 wire, from the demagnetised iron in the helix, in the same 

 direction as the battery current. The action of the bundle of 

 iron wires in the helix of copper wire depends upon the fact 

 that they become a temporary magnet while the electricity is 

 flowing. On breaking contact the poles are suddenly reversed^ 

 the reflex wave flows along the coil, and the magnetism is de- 

 stroyed. 



The secondary coil machine represents two contiguous, but 

 isolated, wires, the first receiving the battery current, and 

 rapidly making and breaking contact by the same means 

 as the primary coil ; the second wire, which is much thinner 

 and longer, reflects or carries off the induced currents, when 

 the ends are brought in contact, so that it conveys alternately 

 one current in one direction, and two in the other. Argu- 

 ments for and against the use of primary and secondary coils 

 in therapeutics were then examined. 



This led to a preliminary inquiry into the difference between 

 quantity and intensity of electricity ; the former represented 

 by the work done in any one cell of a series of cells, not in 

 the aggregate amount done in the whole number ; the latter, 

 by the number of cells. To quantity belongs the power of 

 heating and magnetizing; to intensity, that of overcoming 

 resistance to the passage of electricity. To the latter shocks 

 and sparks belong, for the skin and air are bad conductors ; 

 and to it also belongs chemical decomposition, by presenting 

 a force more powerful than the affinity which binds the 



