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Last update 12/09/2010


Antique Bullet Chronograph

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bullets chronographed since 05/27/07
Sometime ago I came across an antique bullet chronograph.  Like many instruments of a past era, it is housed in a gorgeous walnut case.  Being a shooter and a fan of antique instruments, I just had to buy it.

When I got it home, the first thing to do, of course was to take it apart.  These photos show what I found.

Like all chronographs, this unit measures a bullet's speed by measuring the time it takes for the bullet to cover a known distance.  Modern chronographs detect the shadow of the bullet and compute and display the speed in feet per second.

This one measures the elapsed time by means of two foil screens.  The bullet cuts the foil trace on each screen.  The instrument displays milliseconds.  One converts to FPS by means of calculations or charts.
Here is the face of the instrument. The vertical lines are the displays.  There are 10 numbers in each slot with a neon light behind each.  
The insides.  BCD counting tube style!  Each of the gray modules is a single digit BCD counter.  In the lower left corner is the 100khz crystal and oscillator tube.

When the bullet breaks the first screen, the crystal signal is gated to the counters.  When the second screen is broken, the counter stops.
Here is the vibrator power supply.  The vibrator chops the 12vdc from a battery into approximately 60 hz power.  The big transformer at the bottom of the picture steps the voltage up to that needed by the tubes.  This photo also shows the fine finish of the chassis.  
A closeup of the counters.  There were companies whose business was making modules like these binary coded decimal counters.  One tube for each bit.  
Here is one module removed from the instrument.  
A view of the internals.  Can you image how that kind of mechanical construction would cost now?

I've seen this same module in mil-spec instruments so the construction technique probably involves meeting a vibration specification.
A view showing how the actual display works.  The number plate at the bottom installs in front of the bank of neon lamps.  One lamp after another illuminates as the counter counts.