Figure 2-6.Chronometric tachometer.
When applied to a rotating shaft, the outer drive shaft of this tachometer runs free until a starting
button is depressed to start the timing element. In figure 2-6, note the starting button beneath the index
finger. The chronometric tachometer retains readings on its dial after its drive shaft has been disengaged
from a rotating shaft and until the pointers are returned to 0 by the reset button (usually the starting
button). The range of a chronometric tachometer is usually from 0 to 10,000 rpm and from 0 to 3,000 feet
per minute (fpm).
The rotation frequencies of recording devices and teletypewriter motors can be measured by the use
of a STROBOSCOPE. The stroboscope is an instrument that allows you to view rotating or reciprocating
objects intermittently and produces the optical effect of a slowing down or stopping motion. For example,
electric fan blades revolving at 1,800 rpm will appear stationary if you look at them under a light that
flashes uniformly 1,800 times per minute. At 1,799 flashes per minute, the blades will appear to rotate
forward at 1 rpm; at 1,801 flashes per minute, they will appear to rotate backward at 1 rpm.
When the flashing rate of the light is adjustable, you can calibrate the control in flashes (or
revolutions) per minute. The stationary image you see when the rate of the lamp and the rotational rate of
a shaft are equal lets you record a very precise speed measurement.
The STROBOTAC (figure 2-7) is an electronic flash device in which the flash duration is very short
(a few millionths of a second). (Table 2-1 contains a description of the controls and indicators shown on
the strobotac in figure 2-7.) Because of this short flash duration, the strobotac can measure very rapid
motion. The box contains a swivel mount with a STROBOTRON LAMP in a reflector, an electronic
pulse generator to control the flashing rate, and a power supply that operates from the ac power line. The
flashing rate is controlled by the large knob; the corresponding speed (rpm) is indicated on an illuminated
dial that is viewed through windows in the knob.