Handbook of High Speed Photography - IET Labs, Inc.

5.2.2 MECHANICAL CONTACTS. Mechanical contacts, the most
easily employed triggering device, are suitable for both nonrepetitive
and repetitive motion. The main drawback of mechanical contacts is
their tendency to produce spurious flashes due to contact bounce. Nor
mally open contacts are less satisfactory than normally closed contacts
because they are more likely to have these bounce problems. Expend
able contacts may be made of wire or aluminum foil in either configura
tion. A few examples of mechanical contacts are: a wire that is broken
by the motion of the subject which, in turn, opens or closes an electri
cal circuit, e.g., a wire carrying current cut by a bullet; two pieces of
aluminum foil, to which current-carrying wires are attached, which the
subject presses together; and a thread attached to the subject used to
pull the firing switch of the camera. There are many other triggering
techniques. Photographers of ballistics often use a paper that has a
metal foil coating on both sides. Triggering occurs when the projectile
passes through the paper and makes contact with both foil coatings.
Figure 5-1. A playing card being split by a .22-calibre rifle bullet traveling at
approximately 1150 feet per second. Note how the slug has tipped and started
to tumble from contact with the card.
This action was photographed with a single high-intensity flash from a 1531-A
Strobotac® electronic stroboscope. The reflector was removed, and the flash
lamp was located about 14 inches from the subject. To trigger the flash, a mi
crophone was placed forv/ard of the rifle muzzle and connected to the Strobotac
through a small voltage amplifier. The camera lens (135-mm Schneider, with a
Kodak PORTA +3 close-up attachment) was set at f/8 and located about 10
inches from the card. (Polaroid Type 47 film, 3-microsecond exposure.)
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