Ballistics - Clocking Bullet Speed
June 07, 2018
William L. Casteel authored "How fastdo she go? and How do you know?" in the October 1970 issue of Guns &Ammo. Casteel summarized the development of various devices that measuredvelocity and noted the challenges faced by mathematicians to create formulasthrough calculus that translated measurements to comparable data. The originalstory is below.
Almost anyone who is interested in finding thevelocity of a bullet today can do so without much bother. We have cheap andrelatively accurate chronographs available, while the professionalballistics laboratories with their sophisticated equipment can makemeasurements of extreme accuracy. But did you ever wonder about the time beforethe transistor and the oscilloscope? Just how did the blackpowder burnersdetermine which Minie ball left the muzzle fastest? Well, men have beenmaking velocity measurements for a long time, and surprisingly accurate onestoo.
Before the general use of mechanical gadgets,mathematicians were using calculus to find the muzzle velocity ofcannons. About 1738 a Swiss named Bernoulli would fire a cannon vertically andcount the second required for the ball to strike the ground. Then bymathematics, determine the muzzle velocity. I often wonder just where he stoodwhile waiting for that ball to come down. The first practical device was theBallistic Pendulum, which was invented by an Englishman, Benjaman Robins. Thiswas a heavy pendulum into which the bullet was fired and which retained thebullet. The velocity could be determined by a formula because thevelocity of the bullet was given to the pendulum, and the pendulum swing couldbe accurately measured. These pendulums have been modified andimproved over the years. When properly used, they are surprisinglyaccurate.
Still men persisted in trying to measuremore precisely the time of bullet flight by gauging it over a short distance.The earliest record of an actual mechanical chronograph that I havebeen able to locate, is that of one invented in 1773 by an experimenter namedMathey. He used high-speed clockworks to rotate a vertical cylinder ofcardboard. The bullet was then fired through the drum, directlyacross the axis. Of course with the drum rotating, the entrance hole had movedoff the line of fire by the time the bullet cut its exit hole. By measuring howfar off the axis the holes were, he could calculate how long the bullet hadbeen inside the drum. His accuracy was not good because at that time there wasno really efficient way to determine the speed of rotation. But the idea wassound and the basis for many later devices such as Grobert's velocitywas determined by measuring the angle of displacement between the two holes.This machine also went through many improvements and finally, using electricmotors to drive the discs and tachometers to adjust the R.P.M., were accurateto about 0.7 percent when used with rifles of 2500 fps or so.
Some devices were made which utilized thebullet's cutting of a string to start a timing sequence. One of these was theBenton Thread Yelocimeter. The big problem here was that, although cutting thefirst thread was easy, it was a little tough to hit the second one with anydegree of certainty. Benton solved this by letting the bullet strike a steelplate which in turn cut the second thread. He used a novel device tomeasure the interval. Two small pendulums were suspended from acommon axle, but held horizontally opposite, 180 degrees apart. Whenthe first string was out, it released one pendulum. The second stringreleased the other. When the two passed they struck a glancing blowwhich marked a calibrated semicircle of paper.
Probably the most significant developmentin ball is tics measurements since Robin's pendulum was when the famousphysicist Wheatstone used the breaking of two wires by the bullet tocontrol an electric current. This was in 1840, and the first Wheatstoneballistic chronograph used the breaking of the first wire to start aspring wound clock and the second turned it off. The accuracy was only fair,but the principle opened the way for an entire generation ofchronographs, and of course is still used today on most of ourelectronic types.
A lot of early chronographs utilized the electriccurrent to control electromagnets. One of the first of these was theNavez pendulum of 1854, invented in France. The pendulum was held in ahorizontal 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 thependulum was stopped.
One of the most popular of theelectromagnet types was theLe Belouenge Chronograph, which was invented in Belgium in1864. The Le Belouenge used a long iron-cored rod covered with soft metal,which was suspended from an electromagnet. When the muzzle wire wascut, the magnet released the rod and it fell free. When the second wire wascut, a magnet released a spring loaded knife blade which struck the rod andmarked it. The device was set up again and both magnets released by a singleswitch. This gave another mark which corresponded to the time delay involved inthe magnets and moving blade. The Breger Chronograph was similar tothe Le Belouenge in principle.
Others used the electromagnets to move a markingdevice such as a pen or stylus on a moving index. Some of these were theBashforth and the Sebert and Beitz. While the Schmidt used themagnets to actuate a clockwork escapement. One called the ElectricClypsydra used the magnets to control the flow of mercury through anorifice.
In order to get away from the time delay of electromagnetsand mechanical devices, some ballisticians started experimenting withthe use of an electric spark to mark the recorder. Most of the early ones usedan induction coil to generate the spark. Delay time was therebyreduced considerably. The first known spark chronograph was theSiemens of 1845. These took many forms and used revolving drums or slidingplates covered with soot, which would show a mark where the sparkstruck it. The Casperson and Noble Chronographs were of this type.Others used a combination of electromagnets and spark, such as the Watkins fallchronograph. Watkins released a weight by electromagnet and when the secondgrid wire was cut, a spark was caused to jump from the falling weight to anadjacent soot-covered column.
The Aberdeen proving grounds developed aspark chronograph which operated on a different principle from that of theinduction coil. Two large condensers were charged from a D.C. current sourceand the projectile was fired through them, which shorted them out. This closedthe circuit and caused the spark to jump to a rotating drum. Later sparkchronographs, such as the one invented by Crantz, focused the light from thespark onto a moving photographic plate or paper. There were other variations ofthe spark chronograph; for instance, the Shultz, Mahiew, andSebert-Bianchi. All of the systems, spark or electromagnet, that useda sliding or rotating recorder, required the precise measurement ofthe moving speed. While the well-known Bashforth chronograph used themarks from an electrically driven clock pendulum for his reference marks, themost common means of obtaining a time reference was by tuning fork.
The vibrations from a tuning fork are absolutelyconstant and served the purpose well. On some machines the stylus was attachedto one leg of the fork and allowed to barely touch the soot of the movingrecorder. The fork could be struck manually, or energized by an alternatingcurrent coil. The vibrations would cause the stylus to trace asine-wave pattern of known frequency on the drum or plate. If a photographicfilm or paper was used as a recorder, a light beam was reflected from a smallmirror which was attached to one leg of the fork and focused onto thefilm.
The galvanometer was one of the earliest instrumentsthat could be used to accurately measure small electric currents. Itwas used in several chronographs. In its earliest form the breakingof the wires was used to control relays which determined the length of time acurrent was allowed to deflect the meter. Later the galvanometer was held in anelectrical balance by two opposing currents and the bullet cut firstone wire, which upset the balance and let the meter start to swing. When thebullet cut the second wire the current was shut off and this did away with thetime delay of the relays. Wheatstone, another famous physicist, inventedthis method. Another chronometer used the cutting of the wires to control thebleeding off of a charged condenser. The galvanometer was then usedto measure the remaining charge. Many of these types were in use at the sametime.
In searching for information about these devices Iran 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 whichwas polarized by a Nicol Prism, the polarized light was then passedthrough a glass container filled with liquid carbondisulfide and wrapped with magnet wire to form a coil. The light thenpassed through another Nicol Prism and by a lens system and a slit, toa moving photographic film disc. The polarizing planes of the prisms wereturned at right angles to each other, so that the light beam had to be rotated90 degrees to pass through the second prism. This was accomplished bypassing a current through the coil, which placed the carbon disulfidein a strong magnetic field, causing what is called a birefringenteffect and displacing the light beam. Shutting off the current to thecoil shut off the light. This in effect formed an electrically operated lightgate that operated in the microsecond range. The timing wave reference wasplaced on the disc by reflecting a light beam from a tuning fork mirror. Icannot find any information on the accuracy of this contraption. But whatimpressed me was that it was invented in 1895.
The chronographs that preceded the counter type wereaccurate enough to be very useful. But most required great precision in thesetting up and use. Also, the calculations required toconvert measurements into velocity figures were often time consumingand complex. This put really accurate velocity measurements out ofthe reach of all but a few professionals.