Motion Picture News (Aug-Oct 1920)

3054 (Equipment Service) M o t i o il Picture N Sqvtiprnert-tx Opera,-txn>fe' Q-uestions Answered The "How" and "Why" of the Mercury Arc Rectifier A Treatise on This Piece of Projection Room Equipment ALTHOUGH the mercury arc rectifier for the larger houses of the motion picture field has been somewhat replaced by the motor generator set yet it is still extensively used for changing alternating to direct current for the smaller theatres. The large number of bulbs in present use for motion picture work is emphasized by the requests received for information on the theory and operation of the mercury arc rectifier or converter. As far as the various types and makes are concerned the' mercury arc rectifier is practically standard. The different manufacturers use different methods of arranging the bulb and its connections and also the switchboard layout and equipment but the fundamental principle of operation of all is the same. Figures 1 and 2 show the necessary connections for the operation of a mercury arc rectifier as a means of obtaining a current of continuous flow in one direction from an alternating current circuit. Additional circuits and apparatus are added in Reactance: Supply — Fig. 1 order to make the rectifier automatic in operation, and these will be discussed later on. The " heart " of the rectifier is the peculiar shaped tube by. which the apparatus is easily recognized. The successful operation is due to what is often called "Valve action" of this tube which allows current to pass in one direction only, namely, from the graphite electrodes to the mercury pool. 1 In Fig. 1, A and B are carbon or graphite electrodes to which the alternating current line is connected. C is a mercury electrode and forms the positive terminal of the D. C. circuit. The negative terminal for this circuit is the center of a reactance placed across the A. C. line. An additional electrode shown at D is connected in the bulb for use in making a contact with the mercury pool ; C to start the operation of the bulb. The fundamental principle of operation depends on the fact that a very high voltage is required to start an arc between terminals A and B. There is also a high resistance to the Reactance Supply ~ X Fig. 2 Fig. 3 Fig. 4 electrode C but this resistance is broken down as soon as the current starts flowing. During the time which the terminal A is positive current will pass from A to C, the path and direction of current being shown in Fig. 1. Half a cycle later when A is negative and B is positive the current will pass from B to C, the path for this condition is shown by Figt 2. No current can pass in other directions because of the high resistance set up. Therefore the current in the arc circuit is continuous and in one direction only. Should the flow of current to the electrode C cease entirely then the high resistance would again be set up and the operation of the rectifier stop. In order to prevent this the negative lead of the D. C. circuit is connected to the center of the reactance coil so that the current lags slightly behind the e.m.f. of the A. C. circuit and there is still a small current flowing when the voltage of the line A is zero and the arc from terminal B is ready to strike. In order to start the rectifier the tube is tilted so that a circuit is made by a mercury bridge between contacts C and D, the tube is then raised and an arc is drawn for half a cycle between these two electrodes. This arc vaporizessome of the mercury and fills the tube withironized vapor such that the resistance of the electrode C is broken down and a flow of elecj tricity takes place from whichever of the twei anodes is positive of the cathode C. All are familiar with a characteristic curve! of alternating current as shown by Figure This figure shows the current flow in line XI and for a 60 cycle circuit the action of current changes from a to e is repeated 60 times per second. The current in terminal A starts in a, {Continued on page 2956) What more can we dor me distinction at the time wnereot we quently seen.