A Speed Indicator

Radio Record, Volume III, Issue 45, 23 May 1930, Page 9

A Speed Indicator

-__eooOoOo" OW many know when their gramophone motor is going at the right spded? And yet this is one of the few tasks that remain to the gramophone enthusiast to do. A motor that revolves too slow or too fast, not only spoils the tone, but especially in the latter case ruins the records. On ‘previous occasions the writer has emphasised this. point. The speed indicators on most machines are not entirely reliable. Although they may have been correct when the instrument, left the manufacturer the . regulator soon loses its true significance and some type of indicator is needed to keep the machine up to scratch. Some commercial makes are obtainable though.many listeners will not wish to go to this expense. The simplest method of determining speed is by the stroboscope-a disc of alternating bars or lines. This disc is placed on the gramophone turntable over 2 record. The machine is set playing

and the speed adjusted until when viewed under electric light the bar or lines appear stationary. The light must be provided by alternating current (the usual) and the number of bars determined by. the frequency. In the "Listeners’ Guide" there is a stroboscope-just ready to be cut out and pasted:on cardboard to perform its useful mission. It is designed for the usual 50-cycle alternating current. For those interested the writer follows out the principle involved. WHEN a regular body, such as a wheel, is in rotation, it will appear to be stationary if it be intermittently illuminated, the instants of illumination being such that the whee! turns through an angle equal to thar between two consecutive spokes be_tween successive illuminations. The same effect would, of course, be produced if the wheel were viewed intermittently. . An example of this effect, familiar to most of us, is furnished by the kinematograph. Sometimes, in the film of a moving car, or other vehicle, the wheel appear not to rotate. This

phenomenon is due to the fact that the wheels turned through the angle between the spokes in the interval between the taking of each successive picture. (Intermittent viewing. ) Tf the wheels turn through two or three (or any other whole number ) times the angle between the spokes, the same effect is.produced. Should the wheel turn, in the time between the taking of two consecutive pictures, through an angle slightly greater than that between the spokes, they will appear to move round slowly in the 4irection of rotation, and in the opposite direction. if the angle turned through be less than that between the spokes. The method of arriving at the number of bars in the indicator is

titi, lets teed | Ge ee ee ae best shown by an example. Suppose the frequency of the mains is 50, the usual for New Zealand, and the speed of the machine is 78, now generally regarded as standard. According to the nature of alternating current the light will be dimmed 100 times @ second as in a complete cycle the cur rent passes the zero lines twice on the upward swing and once on the downward. If the number of lines on the disc were such that it turned through the angle between two consecutive lines in 1/100th of a second the disc would appear stationary. The same would take place if.any complete number of the bars were traversed in one internal of full illumination-in our case 1/100th of a second or for any frequency 1/2f. where f. is the frequency of the mains in seconds. Now where R = revs. per minute of the gramophone disc (78), N == the number of lines on the disc, and R = R/60 revs per second. = R x 3860 degrees a second. 60

In 1/2f. seconds the lines has moved through. R xX 860 degrees 60 x 2f,° Therefore, the full 360 degrees of the dise must be split into 60 X 2f. + R spaces. : ' — 420f. -- R sectors or lines which in a circle are equal in our example = 120 X 50 + 78 = 7. But any multiple of this number can be taken, so by doubling the num ber of lines and filling in between each alternate pair, the lines can be converted into bars. For those with a great deal of patience and draughtman’s ability making a stroboscope is not a particularly difficult task. For others it is not so simple.