Ballistic measurement systems: Measuring Time Interval (1950 – 1970)

Measuring Time Interval (1950 – 1970)

The advent of digital techniques in the development of counters opened up a new possibility for accurate measurements in different research areas. This concerned in particular the determination of frequency or time interval between two consecutive events. Because commercial equipment was initially unavailable and/or did not comply with military requirements, the TNO Physics Laboratory developed between 1950 and 1970 three generations of time interval meters (chronographs) for the Royal Netherlands Army Testing Force (CvP). These meters were used when measuring flight times and when determining the speed of projectiles.

Time interval meter with electronic valves

Immediately after the war, the Meetgebouw used army dump components to create a power supply for the Le Boulengé chronograph present at the CvP. Paul-Emile le Boulengé was a major in the Belgian army who developed the principle of that chronograph for artillery tests around 1864. The Le Boulengé chronograph consists of two closed electrical circuits, each with a battery, a wireframe and an electromagnet. One electromagnet holds a long iron bar, the other one a short bar. When a projectile breaks the wires of the first wireframe, the long bar will drop. If the projectile passes the second wireframe at, for example, 50 meters, the short bar drops. This hits a knife which creates a notch in the long bar. The location of that notch (height H) is a measure of the time interval.

Mechanical schematic of the Le Boulengé chronograph

Electrical schematic of the Le Boulengé chronograph — The electrical schematic of the Le Boulengé chronograph (W1 and W2 denote the wireframes)

Le Boulengé chronograph (donated by the CvP)

According to a 1950 report by DMKL/AB, the Le Boulengé chronograph did not always work as desired. Besides, measuring with a Le Boulengé chronograph was time-consuming. The CvP, therefore, wanted to switch to electronic chronographs. The first chronograph was built in 1950. It was based on electron tubes. The chronograph worked at 1 MHz, a very high clock frequency that was unparalleled for that time. The time interval was measured in units of 1 microsecond to a maximum of 1 second. A start signal started the counter, a stop signal stopped the counter. The readout is equipped with neon lights.
These time meters were also manufactured in large numbers by the TNO Physics Laboratory for other clients.

Time interval meter with transistors

This model, taken into use in 1962, measures in units of 1, 10 or 100 microseconds during 1, 10 or 100 seconds respectively. The numerical display uses digivisors, which are rotational reel meters with number plates 0 to 9, one of which is projected onto a frosted glass. [see E.A.C Digivisor Mark.2]

Time interval meter with integrated circuits

This time interval meter was in use from 1970 on. Time units 0.1, 1 or 10 microseconds for 1, 10 or 100 seconds respectively. The readout was with gas discharge numeral tubes.

Photoelectric detectors

Photoelectric detectors have been developed for starting and stopping time interval meters. Two of these detectors detect the passing projectiles and pass the signal to a time interval meter.

Photoelectric detector. The detector has many adjustment possibilities for the fan-shaped detection beam: gap width, gap length and with a zoom lens, the slit height setting

Measurement windows for projectiles with various ballistic measuring instruments under the windows. Each of the windows at fixed distances contain a single winding coil which detected magnetised projectiles. — Measurement windows for projectiles with various ballistic measuring instruments under the windows. Each of the windows at fixed distances contains a single winding coil that detects magnetised projectiles.

To calculate the speed decrease of a projectile, several measurement windows with a single winding coil were placed at fixed distances of 10 meters. The coils were chained in a single circuit. A special amplifier, the Delta-V (no longer present), amplified the signal of that circuit. The amplified signal was converted into impulses that were sent to a maximum of eight chronographs. Measurement window 1 started counter 1. Measurement window 2 started counter 2 and stopped counter 1, and so on.