170 On the Length of Electric Sparks. [Jan. 13, 



will be possible to obtain a current has yet to be determined with a 

 larger discharger now in course of construction *. 



We have alluded to the resistance offered by vacuum-tubes. At first we 

 experienced considerable difficulty in measuring it. For example, when 

 in a Wheatstone's bridge the resistance o£ the tube was balanced by 

 inserted resistances, the galvanometer could only for a short time be 

 brought to rest, and it was then found that the cause of this was that 

 the tubes rapidly increased in resistance as the current passed. After a 

 time, however, they recovered their original resistance, sometimes rapidly, 

 sometimes only after the lapse of days. The resistances were found not 

 to be dependent on the length of tube, but to a great extent on their 

 bore, capillary tubes offering a considerable resistance. Ultimately it 

 was found that it was better to discard the indications of the galva- 

 nometer, and to rely solely on the appearance of a luminosity in the tubes 

 placed on one side of Wheatstone's bridge as soon as the insertion of a 

 balancing resistance was made in the other. 



Later on we hope to have the honour of sending to the Society a more 

 detailed statement of our experiments in support of those now quoted, 

 and in confirmation of our former paper on the cause of stratification in 

 electric discharges in vacuo. 



In conclusion we venture to draw attention to the following consequences 

 of the law of the length of spark being dependent on the ratio of the square 

 of the number of cells of a voltaic battery, in the event of its being con- 

 firmed by experiment. Taking as a basis the spark with 600 cells of the 

 rod-chloride-of-silver battery = 0*0033 inch, a unit of 1000 such cells 



,a • i n -0033x1000^ nAAQ1 „. , 

 would give a spark or 7^2 = 0*0091oo men, 



one hundred units (100,000) a spark of 91*66 inches, 



a thousand units (1,000,000) „ 9166 inches =764 feet nearly, 



whereas a single cell would have a striking-distance of 109 000 000 of an 

 inch only. As far as our own experiments have gone the law has been 

 confirmed ; and although a million cells will probably never be made, a 

 hundred thousand come within the range of experimental possibility. 



January 13, 1876. 



Dr. J. DALTON HOOKER, C.B., President, in the Chair. 

 The following Paper was read : — 



* Postscript, Jan. 8th.— A current was obtained with the negative point distant 

 51 inches from a positive plate 6 inches in diameter. 



