The Oerlikon lamp, which is designed to work with constant potential, is shown partly in section in Fig. 8. There is only one solenoid, A, through which all the current passes, and whose action is to strike the arc and maintain the current constant. The soft iron core, C, is suspended from the inside of the tube, T, in which it has an up and down movement checked by an air piston in the tube. An end elevation of the brake wheels and solenoid is given in Fig. 9, where it will be seen that the spindle carrying these wheels also carries between them a pinion engaging with the rack rod, R. The top carbon attached to the rack rod falls by its own weight, and is therefore in contact with the lower carbon before the lamp is switched in circuit. When this is done the core is instantly magnetized, and attracted to the soft iron brake wheels, which it holds firmly. The air cushion in the tube prevents the core being drawn up until it has fairly gripped the sides of the wheels. The subsequent raising of the core therefore turns the wheels, raises the rack rod, and strikes the arc. The feed is operated by the weakening of the magnetic field of the coil, which causes the core to lose its grip of the wheels, and allows the top carbon to descend. The catch, L, Fig. 8, has a lateral play, and serves to engage in the teeth of the rack rod, so as to prevent its falling when being trimmed. Each carbon when in position is held against two rectangular guide bars by the pressure of a wire spring--see figure. In this way the carbon is pressed against two parallel knife edges, and is therefore always in true alignment. The action of the lamp is very simple, the working parts are few and solidly constructed, and the regulation, as exhibited by the lamps running in the galleries, is exceptionally steady.