Page:Elementary Text-book of Physics (Anthony, 1897).djvu/315

§ 264] action of the machine. This improvement is usually effected by attaching disks of tin-foil at equal distances from each other on one face of a glass -wheel, so that, as the wheel revolves, they pass the contact points in succession.

Another induction machine, invented by Holtz, differs in plan from the one just described in that the metallic carriers are replaced by a revolving glass plate, and the two metallic inductor plates by a fixed glass plate. In the fixed plate are cut two openings, diametrically opposite. Near these openings, and placed symmetrically with respect to them, are fixed upon the back of the plate two paper sectors or armatures, terminating in points which project into the openings. In front of the revolving plate and opposite the ends of the armatures nearest the openings are the combs of two prime conductors. Opposite the other ends of the armatures, and also in front of the revolving wheel, are two other combs joined together by a cross-bar.

In order to set this machine in operation, one of the paper armatures must be charged from some outside source. The surface of the revolving plate performs the functions of the carriers, in the induction machine already explained. The armatures take the place of the inductors, and the points in which they terminate serve the same purpose as the contact points in connection with the inductors. The explanation of the action of this machine is, in general, similar to that already given. The effect of the combs joined by the cross-bar is equivalent to joining to ground that portion of the outside face of the revolving plate which is passing under them.

263. Energy of a System of Charged Bodies.—If the charge on a body be changed, the potential at every point in the field changes in the same proportion. To obtain the energy of a system of charged bodies we may apply the method used in § 248 to obtain the energy of a system of magnets. If $$Q$$ represent the charge of one of the bodies and $$V$$ its potential, the energy of the system is given by $$\tfrac{1}{2} \textstyle \sum \displaystyle QV.$$

264. Strain in the Dielectric.—An instructive experiment