The Instrument Page

What more delicious instrument is there than the Morse Code Key?!

Instruments are not just electric meters, although a lot of electric meters are 'instruments'. Not all, some are just junk. I will start out the page with electric meters!

The so called D'Arsonval system is by far the most common. Just about all instruments here, but for the moving vane and the electrostatic instruments has this on one version or another. A coil, usually with many turns of a fine Copper wire, is suspended in bearings or by thin bands. Inside the coil is a core of Iron and outside, the poles of a magnet. Sometimes (like in the picture below) the magnet is the central body and a ring of Iron is on the outside. A more compact design.

At "A" we can see this central body, a magnet in this case, and the ring of Iron is marked with a "C". The air gap between them, "B", is the space in which the coil moves. The coil is visible just next to "D", under some solder that is the counterweight to the needle at the upper left. In the center, above "A", is a nut and a screw head visible. This is the adjustable top bearing. To the left of "E" is a little spring soldered in. It spirals clockwise in around the center. Underneath the central body is another similar spring and an adjustable nearing. These springs carry the current to the coil. The central magnet in this instrument must be oriented with the poles at 12 o'clock and 6 o'clock. As current flows in the proper direction through the coil, the magnetic field will cause to move clockwise in the bearings until the force from the springs balances the magnetic force. The needle follows, and a reading can be had from its position over the dial.

Here: a meter that has no needle! It uses a beam of light instead. This has some advantages. It means less mass to move and balance. The moving coil in the meter has just a little mirror attached. When the coil rotates in the magnetic field, from permanent magnets usually, the mirror is turned and a spot of light from a built-in lamp moves along the dial. It looks good too!

An added advantage is of course the doubling of sensitivity one gain from the fact that the reflected light moves by twice the angle of the moving coil and its mirror. Just about all light spot galvanometers have a moving coil suspended in thin filaments as opposed to the bearings of an d'Arsonval system. These filaments also provide the torque against which the coil turns. A very sensitive system can be built this way. The meter above has 1 µA for full deflection on it's most sensitive range and 1 A on the least sensitive.

A drawback with an instrument of this kind is that it can be overloaded and nobody would know if the lamp is not turned on, or off scale. Therefore: never leave it (or any other instrument) on a sensitive range, and for really delicate instruments: do not leave them connected to anything. I have repaired instruments for three years, and many of them would never have needed repair had the simple rules above been followed. Some could not even be repaired. One major mistake is to connect a meter with a dial reading, for example, 0 - 5000 V to a high voltage. Such movements often use external resistors. It is usually mentioned on the dial, but make sure before you connect it to anything!

The reading is then the true RMS value of the measured unit! This is necessary for to determine power in a load when the waveform is not exactly sinusoidal. A gasoline generator may or may not have a good sine shape of its output. Linear loads, like light bulbs and heaters, respond to the RMS (Root Mean Square) of the voltage or current. The instruments above are a voltmeter for 0 - 150 V (AC or DC then of course) with an inaccuracy of 0.25% and an Ampérè meter for 0 - 2.5 - 5 A ± 0.2%

So it is also a "True RMS" meter. DC and AC up to at least 500 kHz. The one pictured above is a 0 - 3 kV meter with good resolution on the scale in the 1.2 - 2.7 kV range. With a guaranteed inaccuracy of ±1% of full scale, ±30V, it is pretty good. The shape of the plates determines the linearity of the dial. The amount of plates and the distance between them determines the voltage range. I have seen as little as 500 V full scale.

It is amazing to see how the meter remains at the measured voltage after it is disconnected! The leakage is only in and on whatever insulators are used throughout the instrument. Dirt and air humidity is probably more a source of leakage in older instruments. A resistance of 10¹⁵ Ohm or more is common. The instrument above has a capacitance of some 15 pF, so it can take several minutes to discharge by itself. Even a good insulated lab cable held in the hand will discharge the meter instantly! The only cable one can touch without affecting the discharge is a Teflon insulated cable. Put the conductor in it to the Bakelite on top of the instrument and the instrument will discharge instantly! I thought Bakelite was an insulator…. Everything is relative.

Sensitive Research did it again! (but many years ago)

Model "88"

At $20 on the flea market, I must have got this one for <1% of the 1956 price! It has the following ranges:

In the lid is a signature by a Louise Miller and a date: Feb. 13, 1956. Inaccuracy is ± 0.5% on DC ranges and ± 0.75% on AC ranges. Pretty good even today. All the AC ranges (voltage and current) use a thermocouple to convert the AC to DC. True RMS thus guaranteed! The reason for so many ranges is that one want to read with the needle on the upper half of the scale for to get the best accuracy.

On the most sensitive range, 1 mV full scale, one division is 10 µV. You can see the difference between 75 and 80 µV for example. A 3½ digit DMM will not even pick this up, and a 4½ digit one will show "8" in the last digit, now and then a blinking "7". One can measure the output direct from a microphone, a Fe-Constantan thermocouple will give a full deflection for 18 K (32°F). I have also had good use of this 1 mV range while trouble shooting printed circuit boards with a short circuit. It is child's play to follow the voltage drop in the traces, even if only 10 mA or so flows in the trace.

Accuracy on AC depends on frequency. It is ±1% in the 50 ~ 60 Hz interval, < ± 1.5% 25 ~ 5000 Hz and < ±4% for 15 ~ 20000 Hz.

The meter on the left is not a meter at all in normal sense. It has a potentiometer on the 0 - 100 pointer shaft, and the small dial is coupled with a mechanical gear for ten turns over this range. A servo motor (400 Hz quadrature phase) is connected to this small hand via more gearing. By varying the phase of the signal to one of the windings, the motor can go in either direction and the slaving pot can balance a bridge for example.

The meter on the right has two movements built into one housing. They are "normal", but on the dial one can estimate "VSWR" from where the needles cross. Voltage Standing Wave Ratio. This is the relation between outgoing wave and the reflected wave from a radio transmitter to the antenna. The reflected power should be as low as possible. Two remote RF detectors in the antenna feed line picks up the signals for the meters.

These are 31.5 mm on the outside.

They are also electro-dynamical instruments. Usually the current consumed by the object to be measured generates the magnetic field, and the voltage across it, in series with resistors, drives another current through the moving coil. V • A = W Right?

The one the left, by the famous instrument maker Goertz in Austria, can handle 130, 260 and 520 V multiplied by 1 and 5 A for ranges from 130 W to 2600 W. Inaccuracy is <± 0.5% around line frequencies.

The beauty on the right (thanks Wayne!), by the even more famous American manufacturer Weston, has 75 and 150 V ranges for the voltage and 5 and 10 A for 375 through 1500 W. The lid is missing, so I do not know the inaccuracy, but I dare to guess better than ±0.5% there too.

It is worth noting that Watt meters are notoriously easy to burn! And expensive to repair. They do not tolerate ANY overload. If it says 130 V or 5 A on a terminal, that's it! Be especially careful with reactive loads! It is perfectly possible to have, say, 130 V and 10 A (in a 5 A winding) and still not read more than half scale! Soon the inside of the instrument will fill with brown smoke, alerting you to the fact that the meter also multiplies by cosine for the phase angle between voltage and current! If this angle is 75.5°, the reading is just ¼ of what it would have been with a resistive load drawing the same current! "Cos-fi" is the so called "Power Factor".

On the left above is a very accurate resistor, built by Hewlett Packard. Between the two "hooks" it has a resistance of 1000.000???? W . Squeezing more than four digits out of this is only possible if the measurement is made with a "4-wire" method, also called Kelvin or Thompson coupling. Furthermore, it is of course only meaningful if the resistor body has sufficient stability. The one above is immersed in an oil bath, and the lid has a hole for a thermometer so correction can be made for temperature variations.

The "hooks" have two screw terminals each. The outermost are connected to a source of current and the innermost ones are the ones across which the voltage drop V = I • R develops. This is called a four terminal resistor. They are used for Primary and Secondary standards.

The picture on the right is a four terminal variable resistor! The big knob has 100 steps, 0.000 to 0.100 Ohm in one milli-W (1/1000) steps. Each of the other switches has 0.1 - 1 - 10 - 100 - 1000 - 10,000 W per "click". So, one can set up 87,654.321 W ! At least, this is the resolution. Only frequency can be measured so accurately. No way are the first decades accurate to 1/100 ppm (part per million)! 0.05% probably. It is still not bad, 500 ppm. One part in 2000. Guild, the Canadian manufacturer wanted a fortune (US$260 If I remember right) for a copy of the manual! More than I paid for the instrument! So I guess I will never know the accuracy of this instrument. The entire cabinet is covered with copper sheet metal on the inside. A contact thermometer and a relay switches on and off heaters in the walls for a constant temperature.

The accuracy of the position is basically some 10 meters (30 ft) but the government "dithers" the signals a little so the potential terrorist will not get too good accuracy. It seems to me, though, that better that 50 m, and often as good as 30 m is not unusual. There are methods around it, differential GPS for example, that results in errors of a meter or so. The government is considering to stop the dithering a few years after year 2000.

While 50 m may not be good enough for to land an airplane in foggy conditions, it is certainly enough for a person driving or walking not to get lost. Considering the size of the world, about 40 million meters around, an error of 40 m is 1 part per million! It is as much as 1 mm to a km or 1.6 mm (1/16 inch) to a mile! Furthermore, it is true both in the east - west direction as well as in the north - south direction. So out of the ~5.1E¹⁴ m² on the Earth, this instrument will tell you roughly which 2000 m² you are on! One part in 2.6E¹¹ or 2.6 ppb (parts per billion). This may not be the proper way to determine accuracy, but what the heck!

Besides, accurate time is also had via this instrument. Clearly to better than 1 s but probably much better than so. I guess it depends more on how the signal is taken from the unit. The LCD is not very fast.

Think about how humanity has lived, sailed, traveled and traded for many thousands of years! Maps were precious and guarded secret documents! The information had been gathered at enormous costs and the potential gains were high as well. Trade routes, passages, rivers, islands were found, but where were they? Which way, and how fast was the ship sailing in the rainy night? How far from the treacherous coast was it? The discoverers of inner Africa, Mongolia, China where were they? Although people had "discovered" the land before, by living there, where on the globe was it exactly? How close to the poles did the explorers come? Some finally reached them, but would it not have been something to be able to say that: "No, the North Pole is 125 meter over in this direction!" Or: "What is that Norwegian flag doing over there?"

In my vivid imagination there are pirate treasures buried in caves on small uninhabited islands! The pirates did not have GPS receivers so some of them never found their way back and the treasures are still waiting to be found! The maps the pirates drew had to be difficult enough to interpret if they fell in the wrong hands. Today, two six or seven digit numbers nails it to within a short walk! Not very romantic!

Check out:

 Another clever feature, besides the nice cabinet that swings out the instrument to the desired position, is a linearizing mechanism for the dial on the right. It's linearity, and accuracy, depends on a curved surface underneath that is adjustable with screws in 8 points around the dial! A <1% error is thus easy to achieve, and wear can be compensated for. This particular bridge has been my friend for just over 20 years now!

 An unusual bridge from General Radio:

The Thurston bridge!

Schematic diagram of a version of this instrument.

I could read a license plate on a car from another, moving, car at 300 m. Once I found a good spot in the windshield! The uneven glass distorts the image. So does warm air above the street, and I think it was the major limitation for how far I could read the license plate. I even made a rail where I mounted the binocular in front of my home video camera. It got heavy, but I could easily follow an airliner on cruising altitude, and could even see the air intakes in the engines. It must have been 30,000 ft (10 km) times cot ~ 30° ~ 20 km (~ 12 miles) distance. The effective focal length would be the 68 mm in the camera tele position times 12 = 816 mm.

To the right: the same setup but seen through the binoculars with the gyro spinning. I had problems rigging up the camera straight behind the eye piece so it is off center by a little.

The bubble and the object to be measured are brought together by means of a semi reflective surface and a moveable prism in the top (outside the airplane) of the instrument. A little mechanical counter in front of the larger knob on the right side serves as a read-out. Accuracy of this instrument is better than ±1 minute of arc (~1852 m on the Earth surface).

View of the armature through the collimator. The distance is about 30 m (100 ft) and one can easily see a mm or so. Remember that one can see more details with the eye than the CCD camera can detect.

The collimator is basically a telescope but with an illuminated hair cross added at focus. This particular one has an objective aperture of 42 mm and a magnification of 40x. It is 24x / inch of aperture.

A collimator differs from a regular telescope in that it has a hair cross, often illuminated by a light bulb, in focus (see the left picture above). thus, using the collimator to look into another optical system, a camera for example, where a mirror has been placed instead of the film, a second image of the hair cross is visible through the collimator. This way the camera lens can be aligned and focused (to infinity) properly.

The collimator above also has a spirit level on top of it. It is the big apparatus on top. This level is sensitive to 10". That is 10 seconds of arc. There is 60' (minutes) in one degree and 60" in one minute of arc. 10" corresponds to 48 mm in a km or 3 inches in a mile. The level is of course just that, leveled.

Switching in the 100x immersion objective instead will of course result in a three-fold magnification again. It may to be noted that the theoretical limit for microscopes operating with light is about 1200x. This due to the wave nature of light. One cannot see things smaller than about one wavelength! A typical wave of light, at about blue-green, is 500 nm (500 E^-9 m). The lines above are 10 µm (10E^-6 m) apart. So, there is some 20 waves of light between each line! Already here are we approaching what can be done with light!

 Here I have removed the eye piece and the camera lens. Using only the 100x microscope objective, the magnification on my screen here is 2000x! This is above at the ~1200x limit for a microscope working with white light and you can see how poor the sharpness has become. Just below the middle on the long line a little "hole" is visible. A blue filter or light source would give a little bit more resolution due to the shorter wavelength.

So: beware of "supermarket microscopes" (and -telescopes) that promise 2000x magnification! Same with the telescopes. A magnification in a telescope of more than 60x per inch free aperture is suspect!  

The most well known, before the calculator, may be the slide ruler. The straight slide ruler is commonly known, but how about a circular one? Here: with a pull-out sheet full of equations and constants. On the reverse side: the Periodic Table. If you are interested in buying one, check out http://www.sphere.bc.ca/ for all kinds of slide rulers, links to other such sites as well as some interesting used instruments.

Real Men's Scale. It can measure pull or compression of up to 10 tons. Something Real Men bring when they go fishing or hunting. With a resolution of a kg (a few pounds). It is a Moorehouse Test Ring. It is read off by plucking the tongue visible in the right picture. Adjusting the dial (that has a fine thread in the stem) will touch the tongue lightly and dampen the oscillation. One reads off the dial, repeats it with a load, then goes into the charts that came with the instrument and read the load.

An indicating dial, to my opinion, would have been better, but I am sure Moorehouse had their reasons. Electric strain gauges could certainly be used as well. They have better sensitivity than one would think. They are, like the indicating dial, sensitive to abuse, so it may be the reason for not using them.

More to come I am afraid, so thanks for your time and check back soon!