CIRCUIT DETAILS.

Now, if the flashing rate of the stroboscope is slowed to

1799 flashes per minute, the dot will be illuminated at a

slightly different position each time the disc revolves,

and the dot will appear to move slowly in the direction of

rotation through 360 and arrive back at its original posi-

tion (3 o `clock) one minute later.

A similar movement, but in a direction opposite the ro -

tat ion of the dot, will be observed if the flashing rate of

the stroboscope is increased to 1801 rpm. If desired, the

rate of apparent movement can be speeded up by further

increases or decreases in the stroboscope flashing rate.

When the image is stopped, the flashing rate of the stroboscope

equals the speed of the moving object and, since the flashing rate is

known, the speed of the object is also known. Thus the stroboscope has

a dual purpose of measuring speed and of apparently slowing down or

stopping rapid motion for observation. The practical significance of the

slow -motion effect is that, since it is a true copy of the high-speed motion,

all irregularities (vibration, torsion, chattering, whip) present in the

high -speed motion can be studied.

3.1.2 SINGLE AND MULTIPLE IMAGES.

Single images will occur at the fundamental speed of the object

under observation, and at predictable submultiple of the fundamental

speed. Multiple images will be observed at various speeds above and

below the fundamental speed. Refer to paragraphs 2.8.1 and 2.8.2.

When the Type 1531 Strobotac is used for observation purposes

only, the ability to distinguish between single and multiple images is

usually not necessary. When making speed measurements, however,

the operator must be able to make this distinction. Generally, odd-

shaped (not symmetrical) objects cause little difficult y. Assume, for

example, a fan with only one blade: the single blade will be seen when

a single image occurs, two blades (1800 apart) will be seen when a double

image occurs, three blades (120 0 apart) will be seen when a triple image

occurs, etc.

But when the object is symmetrical in shape (fan with 4 blades, for

example), multiple images cannot always be distinguished from a single

image. This difficult y is easily overcome; simply upset the symmetry

of the object by applying a reference mark with pencil, paint, chalk, tape,

3-2. CIRCUIT DETAILS.

3.2.1 G E N E R A L .

The Type 1531 Strobotac consists basically of a strobotron, an

oscillator to determine the flashing rate of the strobotron, and a power