204 " ELEMENTS OF ELECTRICITY AND MAGNETISM. 



electromotive force between the hollow metal vessel and the 

 walls of the room. In the Toepler-Holtz machine and in the 

 Wimshurst machine, metal carriers are fixed to a rotating glass 

 disk or disks so that at one part of their path these carriers 

 become charged by influence and at another part of their path 

 they pass between two pieces of metal which act like the hollow 

 metal vessel in Figs. 118 to 121, thus combining the principle 

 of the electrical doubler with the principle of the electrophorus. 

 The inducing charge (which corresponds to the charge on the 

 rosin plate of the electrophorus) in the Toepler-Holtz machine 

 and in the Wimshurst machine is generated by the machine itself. 

 Reversibility of influence machines. The Toepler-Holtz ma- 

 chine and the Wimshurst machine may be used as electric gen- 

 erators as described below, in which case they must be supplied 

 with mechanical power and they deliver electrical charge at high 

 electromotive force; or they may be used as electric motors in 

 which case they must be supplied with electric charge at high 

 electromotive force from some outside source, and they deliver 

 mechanical power. Thus, a very large Toepler-Holtz machine 

 driven at high speed may deliver a steady current of o.ooi of an 

 ampere (one thousandth of a coulomb of charge per second) at an 

 electromotive force of, say, 100,000 volts. This corresponds to 

 an output of 100 watts of power, and if the friction losses in a 

 second similar machine are very small, the second machine may 

 be driven as a motor. 



114. The Toepler-Holtz machine. A general view of the 

 Toepler-Holtz machine is shown in Fig. 130. It is difficult to 

 show in a diagram the essential features of such a machine in 

 which the carriers are arranged on a glass disk. Figure 131 

 shows a possible form of Toepler-Holtz machine in which the 

 carriers are fixed to a rotating glass cylinder which is surrounded 

 by a stationary glass cylinder upon which the " inductors " 

 AA and BB (which carry the inducing charges) are supported. 

 The neutralizing rod is a stationary metal rod with metal brushes 

 at its ends, and the figure shows the metal brushes 2 and 4 in 



