Oscilloscopes are especially magical because they translate the abstract world of electronics into something you can visualize. These days, a scope is likely to use an LCD or another kind of flat electronic display, but the gold standard for many years was the ubiquitous CRT (cathode ray tube). Historically, though, CRTs were not very common in the early days of electronics and radio. What we think of as a CRT didn’t really show up until 1931, although if you could draw a high vacuum and provide 30 kV, there were tubes as early as 1919. But there was a lot of electronics work done well before that, so how did early scientists visualize electric current? You might think the answer is “they didn’t,” but that’s not true. We are spoiled today with high-resolution electronic displays, but our grandfathers were clever and used what they had to visualize electronics.

Keep in mind, you couldn’t even get an electronic amplifier until the early 1900s (something we’ve talked about before). The earliest way to get a visual idea of what was happening in a circuit was purely a manual process. You would make measurements and draw your readings on a piece of graph paper.

This sounds crazy, but the trick was to use a spinning rotor with a contact and a matching but movable contact. You’d move the contact to examine different points on the standing wave using a galvanometer (basically a compass with a coil of wire). The speed of the rotor would essentially be your sweep and you would hope the rotating contact was in sync with your waveform.

As you might expect, this was pretty crude and relied on the waveform being constant because your first and last measurement would be many cycles apart. The figure above shows Jules François Joubert’s system for semi-automating the processes which showed up in books around 1915 or so. In fact, there were several variations and you can read more about how it all worked in chapter 63 of Hawkins Electrical Guide.

Automation

Hawkins describes two ways to obtain a waveform: the step-by-step methods like Joubert’s, and the constant-recording method which includes cathode ray tubes, glow lights, moving iron, moving coil and hot wire methods. Remember, a cathode ray tube, at the time, was essentially a laboratory curiosity and required a lot of ancillary equipment and know-how to operate.

One automation attempt was to use the Hospitalier ondograph — a sort of automated Joubert apparatus. While not a catchy name, the ondograph used a synchronous motor geared so that the rotating contact makes one less revolution per minute than the motor does. On each pass, a condenser (what we call a capacitor) would charge to the measured value on the contact which then discharged into a galvanometer with a pen like the one to the left.

Another system involved connecting a mirror to the galvanometer and using a “falling plate” camera. A photographic plate (piece of film, essentially) would slide down under the force of …read more

Source:: Hackaday