Phenomenon of Motion. 535 



length. The horizontal part of the tube was a foot long, so 

 that the thread had to move over a space of 8 inches ; it took, 

 on the average, two to three seconds, according to the intensity 

 of the current. 



As soon as the thread begins to move, it changes its shape. It 

 becomes considerably longer. In my experiments the elongation 

 amounted to an inch. If the current be only allowed to act in- 

 termittently, it is observed that at first the anterior end (that is, 

 the one towards the positive end of the tube) begins to move, 

 while the posterior one remains stationary. When the current 

 is stopped, the posterior end draws itself after. The motion of 

 the thread has in this case a great similarity to that of a leech. 



It is also observed, during the movement of the thread, that 

 on its surface it is in an oscillating condition ; for a number of 

 fine lines are observed, which obviously originate in the reflexion 

 of light from so many undulating crests and hollows*. 



The phenomenon of motion in question is by no means re- 

 stricted to small quantities of mercury ; on the contrary, small 

 quantities of mercury do not stir, probably owing to very great 

 adhesion to the side of the tube. If, by suitable shaking, small 

 portions, of 2, 3, or 4 lines in length, be separated from the pos- 

 terior part of the thread, on the passage of the current it is ob- 

 served that these remain at rest, while the larger remainder exe- 

 cutes its motion as before. 



In the tube used, the mass of mercury set in motion amounted 

 to an ounce. I doubt not that in a larger tube it might have 

 amounted to a pound or more, for in my opinion the moving 

 force is partly in the mercury itself. 



For the production of this phenomenon it is not at all neces- 

 sary to have an intense current or a great quantity of electricity. 

 The current of a Holtz's machine is feeble compared with that 

 of an induction-coil or a voltaic pile. Yet it can be materially 

 weakened and still produce its action apparently as strong as 

 without this enfeeblement. Before entering the exhausted tube 

 I passed it successively through air for a distance of an inch, 

 through a column of water 8 inches long, and through the wire 



* If during the continuance of the current the tube be gently pressed be- 

 tween the fingers, a trembling motion is perceived, which, however, is more 

 electrical than purely mechanical, for the tube acquires polar electricity. 

 If during the passage of the current an electroscope be brought within half 

 an inch, it diverges with positive electricity at the positive end, with ne- 

 gative at the negative end. Singularly enough, this divergence, at any rate 

 in my machine, is not constant, but continually varying. I am inclined to 

 ascribe this to the circumstance that the rotating disk of this machine is 

 not quite plane, so that it is sometimes nearer and sometimes more distant 

 from the stationary disk, by which the current necessarily undergoes a 

 slight variation. 



