LARGE DOUBLE-CURRENT VOLTA-FARADIC APRARATUR. 255 



a manner tliat the axis of the latter was perpendicular to the mag- 

 netic meridian, the needle was seen to make a considerable devia- 

 tion at the moment when the core became magnetic as the circuit 

 was closed. It underwent no further movement, not even the 

 slightest oscillation, when the cylinders were pushed home, either 

 together or alternately. This experiment proved that, if the 

 magnetization was at all diminished by the action of the covering 

 tubes, the diminution was, at most, extremely slight. Neither could 

 I discover any diminution of magnetism by testing the power of 

 the electro-magnet to carry different weights, when tlie coil was 

 under or exempt from the influence of the tubes. It was only by 

 the more or less rapid and noisy vibrations of the trembler, and by 

 the diminished brilliance of the passing spark, that I could deter- 

 mine that the magnetization of the central iron core was really 

 diminished by the tubes. This influence is extremely small on 

 the part of the external tube, by which the noise of the trembler is 

 very little diminished, even when the coil is entirely covered ; but 

 the internal tube, on the contrary, diminishes the magnetization 

 sufi&ciently to impede the motion of the trembler, and to slacken 

 the rapidity of its beats. 



Since I have shown that the copper tubes, placed within or with- 

 out the coil, influence so feebly the magnetization of the soft iron 

 at its centre, it follows that their neutralizing power must be 

 exerted directly upon the induced currents. This view, in fact, is 

 fully confirmed by experience ; and its accuracy may easily be 

 shown by repeating the foregoing experiments with a coil from 

 which the central core has been withdrawn. We may then see 

 that the tubes act upon the induced currents precisely as when the 

 core is in its place ; that is to say, they neutralise them precisely 

 in proportion as they are pushed onwards into or over the coil. It 

 is not only the physiological power of the current that is then 

 diminished, but also the brilliancy of the sparks produced by the 

 intermissions, that is, by the calorific action of the current. 

 Evidently also, in this instance, the tension of the induced current 

 is diminished, as may be shown by various experiments, and, 

 amongst others, by the thickness of the layer of water through 

 which it will pass, and of which it will overcome more or less of the 

 resistance, according to the position of the tubes. (It is unneces- 

 sary to say that, from the fact of the absence of the central core, 

 the intensity of the currents to be graduated by the tubes is much 

 less great.) 



Each of the tubes exerts a special influence upon the current 

 to which it is nearest ; tlius, when the central coil is deprived of 

 the soft iron core, the action of the inner tube is very powerful 



