2 The Hon. J. W. Strutt on some Electromagnetic Phenomena 



whose electromotive force is insufficient to decompose water, be- 

 comes competent to do so by the intervention of a coil or elec- 

 tromagnet. Thus, let the primary wire of a Ruhmkorff coil be 

 connected in the usual manner with the battery, and the elec- 

 trodes of the voltameter (which may consist of a test-tube con- 

 taining dilute sulphuric acid into which dip platinum wires) 

 with the points where in the ordinary use of the instru- 

 ment the contact is made and broken. There will thus be 

 always a complete conducting circuit through the voltameter; 

 but when the contact is made the voltameter will be shunted, and 

 the poles of the battery joined by metal. Now when the shunt 

 is open the battery is unable to send a steady current through 

 the voltameter, because, as has been shown by Thomson, the 

 mechanical value of the chemical action in the battery corre- 

 sponding to the passage of any quantity of electricity is less 

 than that required for the decomposition of the water in the 

 voltameter. When, however, the shunt is closed, a current es- 

 tablishes itself gradually in the coil, where there is no permanent 

 opposing electromotive force, and after the lapse of a fraction of a 

 second reaches its full value as given by Ohm's law. If the con- 

 tact be now broken, there is a momentary current through the vol- 

 tameter, which causes bubbles of gas to appear on the electrodes, 

 and which is often (but not, I think, well) called the extra cur- 

 rent. Allowing the rheotome to act freely we get a steady 

 evolution of gas. 



To this electrical apparatus Montgolfier's hydraulic ram is 

 closely analogous. The latter, it will be remembered, is a ma- 

 chine in which the power of a considerable quantity of water 

 falling a small height is used to raise a portion of the water to a 

 height twenty or thirty times as great. The body of water from 

 the reservoir flows down a closed channel to the place of discharge, 

 which can be suddenly closed with a valve. When this takes 

 place, the moving mass by its momentum is able for a time to 

 overcome a pressure many times greater than that to which it 

 owes its own motion, and so to force a portion of itself to a con- 

 siderable height through a suitably placed pipe. Just as the 

 electromotive force of the battery is unable directly to overcome 

 the opposing polarization in the voltameter, so of course the 

 small pressure due to the fall cannot lift a valve pressed down 

 by a greater. But when an independent passage is opened, the 

 water (or electricity) begins to flow with a motion which con- 

 tinues to accelerate until the moving force is balanced by fric- 

 tion (resistance), and then remains steady. At the moment the 

 discharge-valve is closed (or, in the electrical problem, the shunt- 

 contact is broken), the water, by its inertia, tends to continue 

 moving, and thus the pressure instantly rises to the value re- 



