and of the Aurora Borealis. 197 



dred thousandth of a second, therefore, the air remained passive 

 while the electrical oscillations took place. During this time 

 it is fair to conclude that the heat produced by the passage of 

 the spark was not sensibly conducted away. If conduction of 

 heat had taken place the electrical resistance of the air path 

 would have been sensibly altered and the path of the dis- 

 charge would have changed in form. Here I think we have 

 an interesting limit to the time it takes atmospheric air to re- 

 spond to the phenomenon of heat conduction. 



I have said that the discharges I employed were powerful 

 both in regard to electromotive force and to quantity. Iron 

 terminals one-quarter of an inch in diameter were raised to a 

 white heat by the continuous passage of the sparks and globules 

 of the melted metal were formed. When the sparks were 

 passed through the secondary of a transformer of about thirty 

 seccohms of self induction, three fifty- volt Edison lamps 

 placed in multiple in the primary of the transformer, which 

 consisted of merely two layers of thick wire, were lighted to 

 full incandescence. The spark from two large glass con- 

 densers of 5000 electrostatic units each, excited by an electri- 

 cal machine, passed through the same step-down transformer 

 barely raised a six-volt lamp to a red heat. The study of the 

 efficiency of step-down transformers in thus transforming 

 transient currents of high potential to transient currents of 

 low potential and comparatively large current enables one to 

 obtain an estimate of the high potential of lightning and of 

 the current which accompanies its fall of potential. Thus if 

 C denotes the current in the lightning discharge and E the 

 electromotive force, C and E' the corresponding quantities 

 in circuit of the primary of the step-down transformer and A 

 the efficiency of the transformer we shall have 



C'E' = ACE 



The element of time and the mode of transformation must 

 be considered in any estimate of the amount of energy in 

 lightning discharges. Although a powerful spark of electricity 

 from two Leyden jars each of 5000 electrostatic units is incapa- 

 ble of decomposing water directly ; yet by its passage through 

 the secondary of a suitable step-down transformer it can decom- 

 pose the water in theprimaiy with great evolution of gases, and 

 it is probable that an ordinary discharge of lightning of a few 

 hundred feet in length could light, for an instant, many thou- 

 sand incandescent lamps if it were properly transformed by 

 means of a step-down transformer. Indeed the ringing of 

 electrical bells and the melting of electrical fuses are of com- 

 mon occurrence during thunder storms and manifest the 

 energy of lightning discharges. During a recent visit at a 



