4 digit resolution, 0.01%, +/- 1 digit accuracy
claimed
Serial Number 275
26 transistors, 4 telephone steppers, 10 steps/sec.
rack mount, 19"x5.25"x16", approx. 15 lbs
Manufactured approximately 1960
It is tempting and even accurate to say that "Cubic Corp. was the third manufacturer of digital voltmeters", but that tells you nothing really, and implies that what went before is just a less-developed version of today.
The Cubic Corp. V-45 reviewed here broke no new ground; it is merely a good example of the state of the art. You can read a decent brief history of the digital voltmeter in Electronic Engineering Times' Design Classics. Very briefly, wartime experience [WW2] drove Non Linear Systems' Andrew Kay to create an instrument that unskilled people could read; NLS' first product was introduced in 1951. The Cubic Corp. V-45 is similar enough to this original. (Take a look at some of NLS' products, shown here in a 1960 two-page advertisement from Electronic Designers Catalog; first page, second page).
First, a brief description of the V-45's operation.
Today's view, embedded as we are in a silicon solution to nearly everything electronic, easily sees these early devices as the same as now, only less good. The view then was quite different... Accuracy wasn't the problem exactly; the Kelvin bridge potentiometer, now about a century old, is capable of pretty good accuracy even by today's standards. The problem was that you had to be highly skilled to use it, and it was not fast.
What Kay did was automate the Kelvin bridge: he first had to analyze what a human operator actually did, and find an unambiguous way to indicate the result. His solution was a big stack of electromechanical relays that switched in known resistances, repeatedly comparing the result against the unknown voltage until the smallest-possible error resulted; the "result" for the human operator was the amount of known resistances switched in.
Besides automating the Kelvin bridge, the unique thing about Kay's voltmeter was how the result was presented to the operator. Rather than reading numbers on a row of dials, results were displayed directly as a left to right line of digits, complete with decimal and sign. This was called generically a "one plane display" (other one-plane displays were miniature projection-lamp systems made by IEE, for example). The V-45's display is more or less identical to Kay's original design; it has four decades of resolution (tens, units, tenths and hundredths of a volt, for example). Each digit represents one stepper relay/bridge resistor, and consists of a stack of ten lucite sheets each engraved with one symbol, "0", "1", "2", etc, and indicates which of the ten resistors in that decade are switched in (eg. each lucite digit is an analog for one resistor). Each lucite sheet has a tiny lamp connected to the stepper relay, and illuminates only one of the ten lucite sheets in the stack at one time. (The V-45 can be seen here displaying 10.983 volts; note that "10" is a discrete symbol value in this system). Additional lucite sheets and contacts on the steppers deal with decimal points and the search algorithm, and is reasonably inscrutable. As the voltmeter operates, you can see and hear the instrument iterating, looking for a solution. While the four decades are viewed left to right in the usual order, the stacked numerals make for an odd "hall of mirrors" effect, since "9" is on the bottom lucite sheet and "0" on the topmost. The effect is visible in the photo above especially in the displayed "9". It takes about two seconds of clattering to reach a result.
26 transistors and four telephone stepper-relays are the active elements that embody the voltmeter functions. The "smarts" are all in the designer's head.
The V-45 is very 1960 -- rectangular, clean modern lines, subdued neutral grey colors, few controls, small by then-current standards. Because it is all-transistorized, it consumes little power and has no tubes; hence the cabinet is fully enclosed, no operator-accessible cover and no heat is produced (30 watts maximum). It is a very spare and clean design, with one big obvious feature: square in the middle of the rectangle of the front panel is the lucite display, about 2" high by 8" wide, with "VOLTMETER" in white 48 point Helvetica over it; under it is the Cubic Corp logo of an earth globe in a wire-frame cube. The few controls and such are pushed to the edges to let the lucite display grab the attention.
Operation is reasonably modern; polarity and range are manually set, banana jacks at standard spacing on the left the inputs. The only oddity by today's standards is the SENSITIVITY control that controls the instruments' sensitivity to tiny voltage changes -- more on this later.
The particular relationship technicians had with their instrumentation is lacking today; in practical terms, good riddance. However it had its fine points. Especially in instrumentation from this period, which encompassed such disparate technologies, there were side effects, not all of which were negative.
The most obvious difference with these voltmeters is audible -- telephone steppers are noisy, an electromagnet advances the relay about five steps per second, audible here in RealAudio format in a sample bench session. In the V-45 they are mounted on rubber isolators, enclosed in an air-tight, foam-lined aluminum box, which is isolated from the chassis on plastic insulators; it is still just less than conversation level. The sound is not per se unpleasant, muffled as it is.
To measure a voltage, you connect the meter to the unknown voltage, fiddle the polarity and range controls. If the unknown voltage is not varying, the meter settles down within two seconds and the result can be read. If the voltage varies, or has noise, then the meter never really stops. On a modern voltmeter this barely matters; if the least-signifigant digit changes rapidly, the other, more-signifigant digits are generally valid. The problem with the V-45-type voltmeter is the logical search process is not symmetrical; if the voltage goes up by one digit the meter adjusts quickly; if it goes lower, then the meter has to change all of the digits, overshooting the goal then iterating down until a balance is found. This can take some time, and it is most annoying. The aforementioned SENSITIVITY control was designed to deal with this, partially; by lessening the sensitivity to small changes in voltage you could get a more stable display.
Still, even with the sensitivity problem, the ease of reading these digital voltmeters far offsets the problems, especially in a repeated-measurement, production-type environment.
The sounds of iteration and balance can have a positive effect; you don't have to watch the instrument to know when something has changed, or reached a result. Multi-sensory interfaces can be wonderful (given these givens); I used to work on old minicomputer systems in which the sounds from the power supply would depend on what kind of instructions were operating; you could run hour-long diagnostic programs and know to look over for an error when you heard an anomalous sound.
The display itself is pleasant to look at; it generates it's own light, adequate in daylight and wonderful at night, and the acceptable viewing angle is better than my LCD voltmeter. The digits are fully formed, not ugly seven-segments or bitty outlines.
Whenever the instrument is on, the whir of the chopper is audible, a soft, faint insect buzz, that varies whenever the instrument is jarred even slightly; the tiny, low-mass rotor jiggles in its bearings, but doesn't seem to affect operation.
The innards are quite clean, most of the volume is consumed by the stepper relay enclosure (front is at the top).
For the record, it's accuracy is within 1% of my modern digital voltmeter; which is in error I don't know. Not bad for a device not calibrated since 1972 (according to an old sticker).
This instrument is a nice bridge between past solutions to problems and the present. It's still recognizably a voltmeter, and you could in fact use it today in a pinch. It's old enough to embody some now-alien paradigms, and visually nice to look at. It is a nice lens with which to view today, to see what was important and what was not. The sensitivity control has disappeared. Display technology has improved only slightly, and at greatly increased complexity (NLS switched to Nixie displays in the 60's). The electronics shrunk to a fraction of a square inch. (Auto polarity and autoranging were in fact built into the first NLS voltmeter; AC volts and an ohmmeter were added by 1953 making for a true DMM, though they were still referred to colloquially as "voltmeters".)
