Ballistics – Clocking Bullet Speed
William L. Casteel authored “How fast do she go? and How do you know?” in the October 1970 issue of Guns & Ammo. Casteel summarized the development of various devices that measured velocity and noted the challenges faced by mathematicians to create formulas through calculus that translated measurements to comparable data. The original story is below.
Almost anyone who is interested in finding the velocity of a bullet today can do so without much bother. We have cheap and relatively accurate chronographs available, while the professional ballistics laboratories with their sophisticated equipment can make measurements of extreme accuracy. But did you ever wonder about the time before the transistor and the oscilloscope? Just how did the blackpowder burners determine which Minie ball left the muzzle fastest? Well, men have been making velocity measurements for a long time, and surprisingly accurate ones too.
Before the general use of mechanical gadgets, mathematicians were using calculus to find the muzzle velocity of cannons. About 1738 a Swiss named Bernoulli would fire a cannon vertically and count the second required for the ball to strike the ground. Then by mathematics, determine the muzzle velocity. I often wonder just where he stood while waiting for that ball to come down. The first practical device was the Ballistic Pendulum, which was invented by an Englishman, Benjaman Robins. This was a heavy pendulum into which the bullet was fired and which retained the bullet. The velocity could be determined by a formula because the velocity of the bullet was given to the pendulum, and the pendulum swing could be accurately measured. These pendulums have been modified and improved over the years. When properly used, they are surprisingly accurate.
Still men persisted in trying to measure more precisely the time of bullet flight by gauging it over a short distance. The earliest record of an actual mechanical chronograph that I have been able to locate, is that of one invented in 1773 by an experimenter named Mathey. He used high-speed clockworks to rotate a vertical cylinder of cardboard. The bullet was then fired through the drum, directly across the axis. Of course with the drum rotating, the entrance hole had moved off the line of fire by the time the bullet cut its exit hole. By measuring how far off the axis the holes were, he could calculate how long the bullet had been inside the drum. His accuracy was not good because at that time there was no really efficient way to determine the speed of rotation. But the idea was sound and the basis for many later devices such as Grobert’s velocity was determined by measuring the angle of displacement between the two holes. This machine also went through many improvements and finally, using electric motors to drive the discs and tachometers to adjust the R.P.M., were accurate to about 0.7 percent when used with rifles of 2500 fps or so.
Some devices were made which utilized the bullet’s cutting of a string to start a timing sequence. One of these was the Benton Thread Yelocimeter. The big problem here was that, although cutting the first thread was easy, it was a little tough to hit the second one with any degree of certainty. Benton solved this by letting the bullet strike a steel plate which in turn cut the second thread. He used a novel device to measure the interval. Two small pendulums were suspended from a common axle, but held horizontally opposite, 180 degrees apart. When the first string was out, it released one pendulum. The second string released the other. When the two passed they struck a glancing blow which marked a calibrated semicircle of paper.
Probably the most significant development in ball is tics measurements since Robin’s pendulum was when the famous physicist Wheatstone used the breaking of two wires by the bullet to control an electric current. This was in 1840, and the first Wheatstone ballistic chronograph used the breaking of the first wire to start a spring wound clock and the second turned it off. The accuracy was only fair, but the principle opened the way for an entire generation of chronographs, and of course is still used today on most of our electronic types.
A lot of early chronographs utilized the electric current to control electromagnets. One of the first of these was the Navez pendulum of 1854, invented in France. The pendulum was held in a horizontal position by an electromagnet. When the first wire was cut, the current was shut off to this magnet and the pendulum started to swing down. When the second wire was cut, another magnet was energized and the pendulum was stopped.
One of the most popular of the electromagnet types was the Le Belouenge Chronograph, which was invented in Belgium in 1864. The Le Belouenge used a long iron-cored rod covered with soft metal, which was suspended from an electromagnet. When the muzzle wire was cut, the magnet released the rod and it fell free. When the second wire was cut, a magnet released a spring loaded knife blade which struck the rod and marked it. The device was set up again and both magnets released by a single switch. This gave another mark which corresponded to the time delay involved in the magnets and moving blade. The Breger Chronograph was similar to the Le Belouenge in principle.
Others used the electromagnets to move a marking device such as a pen or stylus on a moving index. Some of these were the Bashforth and the Sebert and Beitz. While the Schmidt used the magnets to actuate a clockwork escapement. One called the Electric Clypsydra used the magnets to control the flow of mercury through an orifice.
In order to get away from the time delay of electromagnets and mechanical devices, some ballisticians started experimenting with the use of an electric spark to mark the recorder. Most of the early ones used an induction coil to generate the spark. Delay time was thereby reduced considerably. The first known spark chronograph was the Siemens of 1845. These took many forms and used revolving drums or sliding plates covered with soot, which would show a mark where the spark struck it. The Casperson and Noble Chronographs were of this type. Others used a combination of electromagnets and spark, such as the Watkins fall chronograph. Watkins released a weight by electromagnet and when the second grid wire was cut, a spark was caused to jump from the falling weight to an adjacent soot-covered column.
The Aberdeen proving grounds developed a spark chronograph which operated on a different principle from that of the induction coil. Two large condensers were charged from a D.C. current source and the projectile was fired through them, which shorted them out. This closed the circuit and caused the spark to jump to a rotating drum. Later spark chronographs, such as the one invented by Crantz, focused the light from the spark onto a moving photographic plate or paper. There were other variations of the spark chronograph; for instance, the Shultz, Mahiew, and Sebert-Bianchi. All of the systems, spark or electromagnet, that used a sliding or rotating recorder, required the precise measurement of the moving speed. While the well-known Bashforth chronograph used the marks from an electrically driven clock pendulum for his reference marks, the most common means of obtaining a time reference was by tuning fork.
The vibrations from a tuning fork are absolutely constant and served the purpose well. On some machines the stylus was attached to one leg of the fork and allowed to barely touch the soot of the moving recorder. The fork could be struck manually, or energized by an alternating current coil. The vibrations would cause the stylus to trace a sine-wave pattern of known frequency on the drum or plate. If a photographic film or paper was used as a recorder, a light beam was reflected from a small mirror which was attached to one leg of the fork and focused onto the film.
The galvanometer was one of the earliest instruments that could be used to accurately measure small electric currents. It was used in several chronographs. In its earliest form the breaking of the wires was used to control relays which determined the length of time a current was allowed to deflect the meter. Later the galvanometer was held in an electrical balance by two opposing currents and the bullet cut first one wire, which upset the balance and let the meter start to swing. When the bullet cut the second wire the current was shut off and this did away with the time delay of the relays. Wheatstone, another famous physicist, invented this method. Another chronometer used the cutting of the wires to control the bleeding off of a charged condenser. The galvanometer was then used to measure the remaining charge. Many of these types were in use at the same time.
In searching for information about these devices I ran across one that is my favorite. It was invented by A.C. Crehore and G.O. Squier. Called the Polarization Photochronograph, it used a light source which was polarized by a Nicol Prism, the polarized light was then passed through a glass container filled with liquid carbon disulfide and wrapped with magnet wire to form a coil. The light then passed through another Nicol Prism and by a lens system and a slit, to a moving photographic film disc. The polarizing planes of the prisms were turned at right angles to each other, so that the light beam had to be rotated 90 degrees to pass through the second prism. This was accomplished by passing a current through the coil, which placed the carbon disulfide in a strong magnetic field, causing what is called a birefringent effect and displacing the light beam. Shutting off the current to the coil shut off the light. This in effect formed an electrically operated light gate that operated in the microsecond range. The timing wave reference was placed on the disc by reflecting a light beam from a tuning fork mirror. I cannot find any information on the accuracy of this contraption. But what impressed me was that it was invented in 1895.
The chronographs that preceded the counter type were accurate enough to be very useful. But most required great precision in the setting up and use. Also, the calculations required to convert measurements into velocity figures were often time consuming and complex. This put really accurate velocity measurements out of the reach of all but a few professionals.
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