DEVELOPMENTS IN COMMUNICATION MATERIALS 249 



life of the switch. This selector consists of an arrangement of closely 

 spaced terminals referred to as the " bank " and a set of rotating brushes 

 contacting with the bank terminals as shown in Fig. 12. Experience 

 in the field indicated that under severe service conditions these selectors 

 have a comparatively short life. As a result of our studies we replaced 

 the brass brushes in the rotor with phosphor bronze, and the brass 

 terminals of the bank with leaded phosphor bronze, a combination which 

 has given approximately four times the life obtainable with brass parts, 

 with corresponding maintenance savings. The reduced wear seems to 

 be due in part at least to a lubricating effect of the lead constitutent in 

 the bank terminals. 



Aluminum alloys have had considerable application to telephone 

 apparatus not only in die castings but in sheet form as diaphragms in 

 certain of the new developments in telephone transmitters and re- 

 ceivers. One of the most interesting of the aluminum alloys is dur- 

 alumin, an alloy of aluminum, copper, silicon and magnesium. This 

 material has about one-third the specific gravity of steel and like steel 

 can be increased in strength by heat treatment in the manufactured 

 form. Our first application of duralumin was as a stretched diaphragm 

 in radio broadcasting transmitters. Here it was necessary to obtain 

 material with as small a mass as possible and with the necessary 

 strength to allow stretching to give a high natural period essential for 

 good quality transmission. The material used in this case was 1.7 

 mils thick and had a tensile strength between 70,000 and 80,000 pounds 

 per square inch. 



Probably one of the most difficult applications of sheet duralumin is 

 to the light valve used in the film method of sound picture recording. 

 The light valve is an electromechanical device actuated by amplified 

 speech currents, and consists of a loop of duralumin tape supported in a 

 plane at right angles to a magnetic field. A view of the light valve is 

 given in Fig. 13 which shows the tape held by two wind-lasses, AA"^, at 

 one end, and wrapped over a spring-supported pulley B at the other. 

 This places the tape under considerable tension. The tape is 6 mils 

 wide and .5 mil thick. The central portion of the loop is supported on 

 insulating bridges just above the face of the pole piece which constitutes 

 the armature of an electromagnet. 



Viewed against the light, the valve appears as a slit 2 mils wide by 256 

 mils long. In operation the amplified speech current is passed through 

 the duralumin tape which, reacting with the magnetic field of the 

 electromagnet causes variations in the width of the slit controlled by 

 the variations in the speech current. The light beam directed toward 

 the film is thus modulated by the slit in accordance with the variations 



