﻿Intelligence and Miscellaneous Articles, 



159 



The very close agreement of these three expressions found by 

 observation (the latter is almost exactly the sum of the two former) 

 gives at the same time a confirmation of the thermoelectromotive 

 fundamental law. The differences are smaller than the possible 

 errors of observation. 



With what accuracy these expressions agree with the observations 

 mr. be seen from the following comparison of some results obtained 

 by observations with those calculated from the formula. 





t. 



e 

 observed. 



e 

 calculated. 



German silver and copper. 

 Copper and iron 



German silver and iron . - 



59-00 

 < 38-27 



32-40 



5107 

 • 34-67 



29-77 

 f66-45 



55-75 

 - 43-07 



32-80 

 1 21-80 



0010156 



0-006348 



0-005335 



0-004561 



0003193 



0-002739 



0-01725 



0-01453 



0-01105 



0-00825 



0-00546 



0010155 



0-006356 



0-005326 



0-004560 



3-003180 



0-002752 



001732 



001441 



001103 



0-00833 



000549 



PoggendorfTs Annalen, No. 11, 1870. 



ON THE DURATION OF FLASHES OF LIGHTNING. 

 BY O. N. ROOD. (FROM A LETTER TO DR. W. GIBBS.) 



After the completion of my first set of experiments on the dura- 

 tion of the discharge of a Leyden jar, I became anxious to make 

 some measurements of the duration of a flash of ordinary lightning, 

 which may be considered equivalent to the discharge of an im- 

 mense jar with an enormous striking-distance. The results of Fed- 

 dersen have shown that the duration of the discharge is increased by 

 an addition to the size of the jar, as well as by augmentation of the 

 striking- distance ; and as both these quantities are so large with a 

 flash of lightning, it was reasonable to expect that the duration of 

 its discharge would be prolonged in some corresponding ratio. 

 During the violent thunder-storm of last August, which occurred in 

 the evening, I happened to be at a house commanding an unob- 

 structed view of the horizon ; and this circumstance, taken in con- 

 nexion with the frequency and proximity of the electrical discharges, 

 induced me, although entirely unprovided with apparatus, to at- 

 tempt a measurement of their duration. A circular disk, 5 inches 

 in diameter, was hastily cut from white cardboard, while a steel 

 shawl-pin served as an axis on which it was made to revolve by con- 

 stantly striking its edge tangentially with the right hand, the pin 

 being held in the left. The maximum velocity attainable in this way 

 was always employed. The general indications at the time were 

 that the rate thus obtained was considerably more uniform than 



