Prof. Magnus on the Changes in the Induced Current, 523 



to 6 millims. pressure by means of the air-pump. Attached to 

 their platinum wires and melted with them into the glass, are 

 aluminium wires, the points of which are from 6 to 40 millimsi. 

 apart. If wires are used made of platinum only, the tubes very 

 soon become covered on their interior surface with a black 

 coating which makes them almost opake. This is not the 

 case when aluminium is employed; hence Geissler* has used 

 this metal in the preparation of his tubes for a long time. Such 

 tubes, when made use of for investigating direction, I shall call 

 test-tubes. 



For the experiments, induced currents only were used. For 

 this purpose two induction-apparatus were at my disposal, made 

 by Ruhmkorff of Paris,— a smaller and older one, the dimensions 

 of which may be considered as known, and a larger one, finished 

 only a few months ago, the induction-wire of which has a length 

 of 40,000 metres, and (without taking into account the silk with 

 which it is covered) a diameter of 0-13 of a millim. 



For both instruments a battery of two of Bunsen's elements 

 was employed, with which the large coil furnished in the open 

 air a spark of from 3 to 4 centims. in length. If it were set in 

 action by a large battery, a spark was obtained which had a 

 length of 39 centims. The apparatus, hov/ever, could not be 

 employed of such a strength for the experiments to be men- 

 tioned presently. '• 



Besides the test-tube, another tube was used, in which were 

 two platinum wires 1 millim. in thickness and rounded at the 

 ends, which wires, by means of a stuffing-box, could be placed 

 at any desired distance from one another. In order to rarefy 

 the air in this tube, it was attached to the air-pump. It is 

 therefore distinguished from the electric egg by its being nar- 

 rower, as well as longer, and admitting of a greater removal 

 of the wires. For the sake of distinction I shall call this tube 

 the air-tube. 



If this air-tube, . together with the test-tube, be inserted in 



* It has often been maintained that particles of platinum are thrown 

 over from the negative to the positive wire. This seems to me to be with- 

 out foundation in the case of induced currents ; for if the discharge takes 

 place through such a tube as has already been described, during a long time 

 and always in the same direction, it becomes covered with a black coating 

 only in that part of the tube where the negative wire is, whilst in the 

 neighbourhood of the positive wire no deposit is perceived even after a 

 much longer time. I believe, therefore, that the black coating origi- 

 nates thus : — the platinum of the negative wire is either volatilized or 

 thrown away from it, but not exactly towards the positive wire. For if the 

 tube contain aluminium wires which are so short that the negative light 

 extends over a part of the platinum wire to which the aluminium is attached, 

 then the black coating is only formed in the neighbourhood of the platinum 

 which is entirely removed from the positive wire, 

 2M2 



