19*3 



NA TURE 



\7uly I, 1880 



EXPERIMENTAL RESEARCHES IN 

 ELECTRICITY^ 



Supplement TO Part III. — The Electric Condition of 

 the Terminals of a Vaaiiiin-Tubc after their connection 

 with the Source of Electricity has been broken 



THE question has been mooted whether there is a 

 polarisation of the terminals of a vacuum-tube after 

 the discharge through it has been interrupted by breaking 

 connection between it and the source of electricity. This 

 question is to be understood in the sense — does tliere 

 exist a chemical polarisation as is the case for instance 

 with the terminals of a voltameter under similar circum- 

 stances ? 



The problem is by no means easy of solution. The 

 authors believe, however, that the few experiments they 

 describe, selected from a long series, will show that there 

 is really no such polarisation of the terminals. 



In the first instance the case of the voltameter is con- 

 sidered ; the arrangement of the apparatus employed in 

 the experiments is shown in Fig. i, where K k' represent 



a special key for breaking connection between any piece 

 of apparatus and the battery, b, and connecting it after- 

 wards instantaneously with a galvanometer, G. The 

 terminals of the battery are attached respectively to the 

 screw-clamps, c and d, carried by an ebonite bar, sup- 

 ported on two ebonite columns, touch-points in metallic 

 contact with c and (/, pass through this bar, so that when 

 the brass springs, ae and If. are allowed to rise and press 

 against these touch-points, the battery is in metallic 

 connection with the screv.'-clamps, a and b, and any piece 

 of apparatus attached to them, as, for example, the volta- 

 meter, V. The voltameter employed has a resistance of 

 20 ohms ; its platinum electrodes are 2'5 inches long and 

 0'025 inch in diameter, and are separated o'2S inch ; it is 

 charged with dilute pure sulphuric acid in the proportion of 

 I volume of acid, sp. gr. I -84, to 8 volumes of water. When 

 the springs arc suddenly pressed down by the fingers 

 placed on the ebonite disks, e and f the connection is 

 broken between the voltameter and the battery, and it is 



' " Experimental Researches on llie Electric Discharge with the Chloride 

 ?, ,w'^^"f,'>''" ''y Warren De La Ru:, W.A., D.C.L., F.R S., and 

 Hugo W. MQIler, Ph.D., F.R.S. 



connected instantaneously with the galvanometer G. The 



galvanometer actually employed was not that shown in 



the figure, but a Thomson galvanometer, whose constant, 



I volt o , 1- ■ • ^ 



C, = r — = i)074 scale divisions. On connecting 



I megohm ° 



a battery of 10 chloride of silver cells with the voltameter 

 for a minute or less, and then suddenly pressing down the 

 springs e and /, there was a deflection to the left, say, of 

 more than 1000 scale divisions, althou^'h the ,;,ig shunt 

 was used to reduce the current through the galvanometer; 

 therefore, the deflection without the shunt would have 

 been more than 1,000,000 divisions. By comparing this 

 deflection with that produced by a half microfarad con- 

 denser, charged with 240 cells, it was ascertained that 

 the deviation produced by the voltameter was equivalent 

 to that of I II microfarads. 



The small condenser shown at C, Fig. i, was substituted 

 for the voltameter ; it is made of a thin plate of glass 2 

 inches square and 001 15 inch thick; the tinfoil coatings 

 being r5 inch square, its capacity was found to be 

 o'ooo55 m.f. When charged with 3,600 cells, and after- 

 wards connected with the Thomson 

 galvanometer through the ^ shunt, by 

 pressing down c and f the deflection 

 was I36'5 divisions to the left; this 

 multiphed by 9'92, the value of the 

 shunt was equivalent to 1,354 divisions. 

 The apparatus, shown in Fig. 2, 

 which was constructed for another 

 object already described,^ was con- 

 nected with a and /' ; it consists essen- 

 tially of two disks, 3'i inches diameter, 

 placed o'i3 inch apart. The capacity 

 of this apparatus, when used as an 

 air-condenser, was determined, and 

 found to be o'oooojS m.f. With 3,600 

 cells no discharge took place, and it 

 merely charged up as a condenser. 

 The deflection produced, when the 

 keys e and f were pressed down, was 

 (without shunt) 150 divisions, still to 

 the left. It is evident, therefore, that 

 the direction of the deviation throws 

 no light on the question, for it is the 

 :ine with the voltameter as with the 

 iidenser. 



Tube 73, containing a residuum of 

 acetylene, was now substituted for the 

 air-condenser ; it is 26'5 inches long 

 and I '5 inch diameter, the distance 

 between the termals 23 inches ; this 

 tube was connected with 3,600 cells, 

 current o'oo6Si W.j The tube potential 

 was found to be 2,980 cells, and its resistance 449,500 

 ohms. On pressing down the springs e and /, so as to 

 break connection with the battery and connect the 

 tube with the galvanometer, there was a deflection of 

 1 1 divisions to the left, the bame as before. The current 

 was only 



-!^ W. 



1874 X 1,000,000 170,000,000 



The diagram. Fig. 3, will illustrate the action of the 

 special key. Fig. i. When the handle is moved to 

 the left, the tube T t' is placed in metallic connection 

 with the battery, whose terminals are shown attached to 

 (■ and ^/(this is equivalent to the springs being allowed to 

 press upwards against c and d, Fig. i) ; when the handle 

 is moved to the right, then the discharge of the battery 

 through tt' ceases, and the terminals of the tubes n and 

 P are connected with the galvanometer, the extremities of 

 whose coil arc attached to e and / (this is equivalent to 

 pressing down the springs in Fig. i). 



On another occasion with the same tube. No. 73, with 



' Phil. Trans., vol. clxxi. p. 76 ; N'atuke, vol. .\xii. p. 153. 



