Lightning Discharges and of the Aurora Borealis. 345 



this length of time every sinuosity in the pilot spark is exactly 

 reproduced. I employed terminals of tin ; and in fig. 1 it 

 will be noticed that a mass of melted and vaporized tin 

 remained suspended in the air for at least three hundred- 

 thousandths of a second before it was dissipated in a comet- 

 like tail. During the three hundred-thousandths of a second, 

 therefore, the air remained passive while the electrical oscil- 

 lations 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 discharge would have 

 changed in form. Here, I think, we have an interesting 

 limit to the time it takes atmospheric air to respond 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 secohms 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 copper wire, were 

 lighted to full incandescence. The spark from two large 

 glass condensers of 5000 electrostatic units each, excited by 

 an electrical machine, and passed through the secondary of 

 the same step-down transformer, barely raised a six-volt lamp 

 in the primary to a red heat. The study of the efficiency of 

 step-down transformers in thus transforming transient cur- 

 rents 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 cur- 

 rent which accompanies its fall of potential. Thus, if C 

 denote the current in the lightning discharge and E the elec- 

 tromotive force, C and E' the corresponding quantities in 

 the circuit of the step-down transformer, A the efficiency of 

 the transformer, we shall have 



(7^= ACE. 



The element of time and the mode of transformation must 

 be considered in any estimate of the amount of energy in a 

 lightning discharge. Although a powerful spark of elec- 

 tricity from two Leyden jars, each of 5000 electrostatic units, 

 is incapable of decomposing water directly, yet by its passage 

 through the secondary of a step-down transformer it can 



Phil. Mag. S. 5. Vol. 36. No. 221. Oct. 1893. 2 A 



