INTERFERENCE FILMS ON TUNGSTEN IAN H. PARSONS London, England
W H I L E colored interference films may he formed on several metals by electrolytic action, a device using tungsten, developed by the writer during an investigation of the anodic behavior of that metal, lends itself particularly well to the demonstration of interference phenomena.
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w' F i g 1. Circuit D*.m
for Colowd Film circuit.
~h~ circuit diagram is shown ir; ~i~~~~ and requires no explanation, the electrolytic cellq,eing shown in Figure 2. A circular tungsten plate a6out two and onehalf inchesiu diameter forms the anode. The electrolyte is 0.06 N sodium hydroxide, protected from carbonation by a layer of mineral oil; the cathode is an iron plate a little smallerthan the anode so that it isinvisiblefrom the front. A glass fish globe serves as a container and possesses the advantage that, being spherical, it magnifies the apparent size of the tungsten plate when filled with electrolyte. It may be added that the back of the tungsten plate should he insulated by a hard rubber backing or other suitable means, and that the device may be operated either with a switch or, preferably, by an automatic circuit breaker with "on" and "off" periods of about 15 seconds each. On closing the circuit a thin film of oxide of tungsten is produced on the anode, the color of this fihn having a most striking appearance. The film has a high resistance and, as the potential across the cell increases, the film becomes thinner. This Change in thickness shows itself as a change of color, all the colors
of the SPeCtm being passed through in succession. When the circuit is broken, the oxide film dissolves in the electrolyte, the colors succeeding one another in the reverse order. It will be found that with an automatic circuit breaker of the type described, a continuous change of color may he obtained. When the tungsten surface is brightly illuminated, the device prbvides a most attractive illustrakion of interferenee phenomena. Should a tungsten disc of the type described not he available, the phenomena can of course be demonstrated using tungsten in another form; thus, a tungsten rod may he employed in a boiling tube. However, the arrangement first described is particularly suitable for display purposes. Three other points of interest may he mentioned in connection with this device. The first is that the rise of potential aeross the electrolytic cell during the brief period of film formation may be clearly shown by a lowwattage lamp; thus, if the l i e voltage he 110 volts and a 25-watt, 110-volt lamp be connected across the cell, on closing the circuit this lamp will gradually come up to full brilliance in about 15 seconds. In the same period a high-wattage lamp in the main circuit will pass from normal illumination to a dull red or even cease to glow. The second point to note is that, by manipulation of an adjustable resistance in the main circuit while the current passes, the film color may be brought to, and held at, any desired color. If, after formation of the film with the circuit closed, the tungsten anode is washed with water as it is withdrawn from the cell, the colored film may then be dried and preserved; such films are quite
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