considered in connexion with the Dynamical Theory. 3 



quired to overcome the weight of the great column of water. 

 The second valve is accordingly opened, and a portion of the 

 water is forced up. Now the electrical current, in virtue of 

 self-induction, can no more be suddenly stopped than the cur- 

 rent of water ; and so in the above experiment the polarization of 

 the voltameter is instantly overcome, and a quantity of electricity 

 passes. 



If no second means of escape were provided for the water in 

 the hydraulic ram, the pipe would in all probability be unable 

 to withstand the shock, and in any case could only do so by 

 yielding within the limits of its elasticity, so as gradually, though 

 of course very quickly, to stop the flow of water. The bursting 

 of the pipe may properly be compared to the passage of a spark 

 at the place where a conductor carrying an electric current is 

 opened. Just as the natural elasticity of the pipe or the com- 

 pressibility of the air in a purposely connected air-vessel greatly 

 diminishes the strain, so the electrical spark may be stopped by 

 connecting the breaking-points with the plates of a condenser, 

 as was done by Fizeau in the induction-coil. Contrary to what 

 might at first sight have been expected, the fall of the pri- 

 mary current is thus rendered more sudden, and the power of 

 the instrument for many purposes increased. Of course the 

 spark is equally prevented when the breaking-points are con- 

 nected by a short conducting circuit, as in our experiment by 

 the voltameter. In fact the energy of the actual motion which 

 exists the moment before contact is broken is in the one case 

 transformed into that of the sound and heafc of the spark, and in 

 the other has its equivalent partly in the potential energy of the 

 decomposed water, partly in the heat generated by the passage 

 of the momentary current in the voltameter branch. 



The experiment will be varied in an instructive manner if we 

 replace the voltameter by a coil (with or without soft iron), ac- 

 cording to the resistance and self-induction of the latter. In 

 order to know the result, we must examine closely what takes 

 place at the moment when contact is broken. The original cur^ 

 rent, on account of its self-induction or inertia, tends to conti- 

 nue. At the same time the inertia in the branch circuit tends 

 to prevent the sudden rise of a current there. A force is thus 

 produced at the breaking- points exactly analogous to the pres- 

 sure between two bodies, which we will suppose inelastic, one 

 of which impinges on the other at rest. The pressure or elec- 

 trical tension continues to vary until the velocities or currents 

 become equal. All this time the motion of each body or cur- 

 rent is opposed by a force of the nature of friction proportional 

 to the velocity or current. Whether this resistance will affect 

 the common value of the currents (or velocities) at the moment 



B2 



