128 



Popular Science Monthly 



This instrument is arranged to use motor 

 drive, as motors are cheap and easily applied 

 and if alternating current is available a 

 constant speed is easily secured. The tone 

 arm and reproducer are to be made of wood, 



Here is shown the horn or sound box beneath 

 the turntable cover and the cabinet fastenings 



which is more easily worked than metal and 

 is generally more desirable. The most 

 recent phonographs placed on the market 

 have a wood tone arm. 



Any type of motor, preferably shunt 

 wound, may be used and should develop 

 only a few hundredths of a horsepower. 

 The motor usually used on an 8-in. fan is 

 ideal and of the right size. The speed 

 should preferably be not over 3,000 

 R. P. M.; but this is not essential as the 

 speed can be controlled by resistance. 

 A motor should be selected which has a 

 uniform rate of speed. The method of 

 mounting a motor of the "globe" type 

 is shown in Fig. 1, in which the motor 

 swings on pivots in a strap-brass frame 

 secured to the top panel, and is held so 

 that its friction wheel bears against the 

 turntable by a* suitably arranged 

 spring. For any other type of motor a 

 suitable mounting must be developed 

 but should always be sufficiently resil- 

 ient to produce enough pressure for 

 driving. In order to insure uniform 

 speed of the turntable the disk is made 

 very heavy, so as to act as a flywheel, 

 driving pulley should be so proportioned as to 

 give a speed normally of 80 revolutions per 

 minute. The motor pulley should be cov- 



ered with a piece of soft rubber tube to cause 

 proper friction. The turntable should be 

 made of cast iron and carefully turned in a 

 lathe. The center pin is forced into the disk 

 and rotates freely in the bushing beneath, 

 which is mounted in the crossbar shown 

 secured to ^he underside of the top. The 

 turntable is 12 in. in diameter and 1 in. 

 thick. 



The top view of the machine with the 

 cover removed is shown in Fig. 2. Here the 

 wooden tone arm is shown and the position 

 of the motor. The two holes in the top are 

 for new and used needles. The turntable is 

 covered with a layer of felt stuck on with 

 shellac. 



The top view. Fig. 3, of the cabinet with 

 the top removed shows the horn and the 

 method of fastening the cabinet by screws 

 through corner pieces, thus preventing them 

 from showing on the outside. 



Three views of the wood tone arm and re- 

 producer are shown in Fig. 4 and Fig. 5. 

 The arm is made of two pieces sawed out 

 roughly as in Fig. 6, then gouged out and 

 sandpapered to make the tone passage. 

 Afterward the two pieces are glued together 

 while firmly clamped in a vise and finally 

 carved down on the outside and polished to 

 the shape shown in Fig. 4. The reproducer 

 sound chamber, Fig. 5, is simple. It consists 

 of a wood back. A, a pair of rubber rings B, 

 the mica or celluloid diaphragm, C, the 

 stylus D, and the clamping ring, E, which 

 should be of brass 1/16 in. thick. Four 1/16 



The 



Plans of the tone arm and reproducer which are 

 easily constructed from wood and metal rings 



bolts arranged as shown secure the parts 

 together. The stylus is best made from a 

 single piece of brass with the aid of a fine- 

 toothed pack saw and a few files. A detail 



