Continued from part 2, the more modern chart recorder.
= = = = =
Moving a pen back and forth requires a fair amount of force. It was always possible with mechanical signals like an aneroid barometer, or strong electrical signals like a telegraph. Morse’s 1840 telegraph used a moving pen. But a delicate signal from a low-level transducer or a nerve couldn’t possibly move a pen. The chart recorder for most signals had to wait for vacuum tube amplifiers.
GenRad’s earliest Oscillograph followed a much older tradition predating phonographs and tubes. Sound is recorded on photo film as a mirror or wire wiggles and reflects light onto the film. The typical oscillograph mounted a miniature mirror on a magnetic galvanometer needle. Note the big permanent magnet surrounding the rotor, which carried a delicate electromagnet winding.
I’ve set up the galvo to read the fungal current between two clover plants, which would certainly be a delicate little current. Happystar is moving the photo slide forward as the reflected light beam creates an image.
= = = = =
Now let’s imagine an oil well logger like Halliburton, applying sound waves of varying frequencies to the ground and monitoring the response to each frequency.
This is similar to the audiometer setup in the previous item, except that the strata and layers of the earth are responding instead of the strata and layers of a cochlea.
Here’s the two GenRad instruments. Below is an oscillator to create the sound, above is the String Oscillograph, a complex and rather weird device that looks much more homemade than the usual stylish GenRad equipment. The oscillograph is strictly old-fashioned, no tubes or electronics, just magnets and wires and mirrors.
= = = = =
The oscillograph on its own, seen from above with the Filmbox open. A light bulb is inside the barrel at right. The light goes through a focusing tube to the aperture in the middle, where it encounters the moving wires. The wires are the real sensor in this device, moving in and out of the big permanent magnet as very small currents pass through the wires. The light then travels leftward to the filmbox where it bounces off the mirror, which is only serving as a scanner here.
= = = = =
The beam leaves the light bulb and passes through the focusing tube toward the aperture:
= = = = =
Looking at the aperture from above, where the beam crosses the two moving wires and creates shadows. Signal is connected to each wire via the binding posts on top, and each wire pulls slightly into the magnetic field when it has current.
= = = = =
The beam with moving shadows then goes through another focusing tube and another aperture to shape it into a horizontal rectangle, and hits the slow-turning scanning mirror.
= = = = =
Finally, shown without the beam, here’s a light pattern striking the photo film, which is stretched across an arc so it records the slow scan as a linear time axis. Instead of the single beam relative to darkness of the mirror galvo, the moving areas are recorded as dark spots.
= = = = =
The action of the moving wires is strange and hard to see in the original diagrams. I’ve tried to understand it so I can explain it. Here’s a view of the module contining the wires, pulled up out of the aperture, as seen from the right. This module appears to be meant for a different setup where it was mounted in a large panel or rack. Its presence here looks jury-rigged. The front of the module has two binding posts for the open end of both wires, and one post at bottom for the common ground. The knob is a screw that pushes a little platform carrying the wires, so you can get them centered in the alidade.
= = = = =
Here’s another view looking leftward from the light bulb, with the module in place, showing an alidade that narrows down the light before the wires. You can see the two wires moving at different frequencies, as their different signals make them more or less attracted to the big magnet on the right.
= = = = =
Now we’re looking into the light tubes from the left end, from inside the filmbox. Another alidade at the left end of the tubes keeps the beam horizontal, so the wires are effectively just dark dots in a horizontal stripe of light.
= = = = =
Finally, the slowly turning mirror scans the slot onto the film. This would paint a minute or so of the signal across the film, leaving a typical time-axis graph. GenRad also made a movie camera that could attach to the top of the filmbox, so you could make much longer graphs.
= = = = =
Continued in part 4.
Polistra's Mill 