How long was cm used as a unit of capacitance?

[ricard]

I'm trying to determine the age of an old 16mm movie projector I have acquired, and one thing that I have noted is that the motor noise suppression capacitors (the motor is a universal motor with pole reversal so that the machine can run in both directions) are labeled in 'cm' and not 'µF'. Well actually, one of them is labeled in both in a curious way.

As far as I can tell the machine dates from somewhere between the 1930s and the 1950s. It is made in Austria by Eumig. In my collection I have various tape recorders dating back to the mid 1950's, but none of them have capacitors labeled in 'cm'.

The capacitors themselves are in the shape of phenolic tubes sealed with black tar, and there is actually more than one capacitor in each package. Their capacitance actually measures quite ok.

One of the capacitors is marked:

20 000 cm 8,5 20 000 cm

and

500/1500V

This one has three leads and measures as roughly 50 + 50nF, which is odd, I would have thought about 20 + 20nF, unless there is severe leakage (not unlikely given its age) throwing the capacitance measurement off; apparently (https://en.wikipedia.org/wiki/Centimetre) when used as a unit of capacitance, 1 cm is 1.113 pF .

I'm guessing that the voltage measurement means 500V AC, 1500V DC.

The other is marked:

250V ? W 2000 cm (I can't make out the number before the W).
0.1µF

It has four leads, measuring roughly 50nF, 50nF and 5nF to the one connected to ground.

The 50nF parts are connected between the respective motor connections and the motor frame (which is isolated from the chassis) , with the 5nF connected between the motor frame and the chassis. The machine was not originally earthed, however, I have now fitted an earthed mains cable as the old one was made of rubber and slowly deteriorating.

(Yes, extreme caution (and an isolation transformer) is employed when plugging this machine in due to its age. Also, the motor mounting screws are accessible from the outside of the machine which is cause for further caution, given the capacitor arrangement and potential failure thereof).

[Herald1360]

Capacitance in cm certainly lasted until early '50s at least. I think it was more of a continental unit being cgs based and not imperial. It may have gone on until the SI units became officially adopted.

[dominicbeesley]

I seem to recall a French Philips set from the 50's having some caps marked in cm.

As to when it stopped being used - at University in the 1990's there were still lecturers and newish text books that used cgs. The worst was an American textbook that would have been good but for the mix of cgs, mks, imperial and British Thermal Units. The clarity of the explanations ruined by constantly having to convert weird and wonderful units - I remember the book costing a fortune, being rarely used and then the binding giving up. I took great pleasure in burning it after my 2nd year exams. The only book I've ever burnt!

[David G4EBT]

A point worth noting is that cms aren’t directly comparable to pF; In the CGS system, 1 cm of capacitance = 1.113×10−12 farads (1.113 x 1 picofarad). Hence, a capacitor marked 100cm = 111.3pF. So 1,000cms would be 1,000 x 1.113 = 1,113 pF, and 10,000 cms = 1,1130pf = .0.0113 uF. Won't matter a jot for larger capacitors, which are usually at least +/- 10% tolerance, but it would matter for ones below say 200pF if used in tuned stages which often call for 1% tolerance, so a variance 11.3% becomes significant.

A clip from wiki of the CGS/MKS systems of measurement:

8-<

The CGS system goes back to a proposal in 1832 by the German mathematician Carl Friedrich Gauss to base a system of absolute units on the three fundamental units of length, mass and time. Gauss chose the units of millimetre, milligram and second.[1] In 1874, it was extended by the British physicists James Clerk Maxwell and William Thomson with a set of electromagnetic units and the selection of centimetre, gram and second and the naming of C.G.S.

The sizes of many CGS units turned out to be inconvenient for practical purposes. For example, many everyday objects are hundreds or thousands of centimetres long, such as humans, rooms and buildings. Thus the CGS system never gained wide general use outside the field of science. Starting in the 1880s, and more significantly by the mid-20th century, CGS was gradually superseded internationally for scientific purposes by the MKS (metre–kilogram–second) system, which in turn developed into the modern SI standard. Since the international adoption of the MKS standard in the 1940s and the SI standard in the 1960s, the technical use of CGS units has gradually declined worldwide, in the United States more slowly than elsewhere.

8-<

https://en.wikipedia.org/wiki/Centim...ystem_of_units

So, given that the MKS system became internationally accepted in the 1940s, I think it's unlikely that capacitors were being marked up in cms into the 1950s. That said, things do linger on - the UK has taken decades for metric to slowly displace imperial measurements – sometimes using the force of law to compel shopkeepers to sell goods by KG/Litre. It wasn't until 1988 that the National Curriculum adopted S.I Units in schools, and not until 1999 did the British Government allow the use of imperial units to lapse, when all goods (apart from beer, cider and milk in returnable containers) had to be priced in metric units by law.

The one thing that always seems to still be quoted in pounds and ounces is the weight of new-born babies!

Hope that's of interest.

[bikerhifinut]

So what does Cm stand for? i can't think how capacity can be defined as a unit of length?
Does the C stand for Coulomb perhaps?

A.

[ms660]

http://web.mit.edu/sahughes/www/8.022/lec06.pdf

Lawrence.

[Lucien Nunes]

In the CGS system capacitance is indeed stated in units of length, but the explanation of this in terms of familiar MKS units is a bit complex.

In the MKS system we relate the electrical units to the kinematic ones (length, time, force) by two constants, the vacuum permittivity ε0 and vacuum permeability μ0. These are defined to make the electrical units numerically consistent and convenient to work with, but they are not just numerical factors. ε0 for example has dimensions of capacitance/distance, ε0=8.85 exp-12 F/m (you can perhaps visualise this as capacitance per unit area multiplied by the distance between plates).

These constants aren't used in CGS, where the electrical quantitites are directly defined in terms of the kinematic ones, and hence differently for electromagnetic units (EMU) and electrostatic ones (ESU). 1F is equivalent to 10exp-9 c² cm (where c is the speed of light) and hence dimensionally different. In fact all the electrical quantities are different, there are statamps for electrostatic measurements and abamps for electromagnetic ones, resistance is measured in seconds per centimetre, etc. etc.

The MKS system uses the artificial construct of the vacuum constants to free us from the tyrrany of these fundamental but highly inconvenient physical definitions of electrical quantities.

[ricard]

Here are pictures of the components in question. Looking at the closeup of the larger one now, it appears that it says

0 220V~ / 1000- 2000cm
0.1µF

rather than what I noted in the first post.

The 0 is there on both caps. It appears that it indicates the common terminal, similar to the way electrolytics are denoted.

Can anyone take a guess at what the '8,50' stands for on the right hand component?

[HamishBoxer]

Possibly the date it was made.

[RF Burn]

Originally Posted by bikerhifinut

So what does Cm stand for? i can't think how capacity can be defined as a unit of length?
Does the C stand for Coulomb perhaps?

The unit is indeed the centimetre (cm) a unit of length. The capacitance of a sphere (in space) is simply its radius in centimetres, as an example the mean radius of our planet, Earth, is 640,000,000 cm and it has a capacitance of close to 700 microfarads.

[wd40addict]

If you quote the model number and/or post a picture of the projector I may be able to tell you what the likely year is.

[julie_m]

The other way of looking at it is that CGS either assumes that ε0 = 1 (and is dimensionless) which gives you the stat units, or that μ0 = 1 (and is dimensionless) which gives you the ab units.

We now know that ε0 and μ0 are not independent, but are related by the equation
ε0 * μ0 = 1 / (c ** 2).
The present definition of the ampere (in terms of force exerted between conductors carrying current; but moves are afoot to replace the definition of the ampere as being a fixed number of electrons per second) effectively fixes
μ0 = 4 * π * 10 ** -7 H / m
and the existing relationship in turn fixes ε0.

[ricard]

Originally Posted by wd40addict

If you quote the model number and/or post a picture of the projector I may be able to tell you what the likely year is.

That would be very helpful. There doesn't seem to be any label or plate indicating the actual model, only the serial number.

From this page here:
http://www.pathefilm.uk/95gear/95geareum.htm
it seems like it would be a P11 or P25, or possibly even the later P26. But the P11 is from 1935 which is a trifle old, and the picture of the P25 looks like it is in lighter colour such as light brown or beige, although the year would make more sense as that model was apparently launched in 1951.

[wd40addict]

Hi Ricard,

I've been digging into this (from a UK perspective) and I think it's most likely a late model P2, probably from 1950. Just like most consumer goods the cine market took some time to recover from WW2. Initially other makers such as Bolex, Specto and Ditmar re-introduced their pre-war machines. Styling did slowly change as before the war black crackle was de rigueur. But this slowly was replaced by bronze, grey etc hammer finish.

Before the war Eumig was distributed in the UK by Nebro, but they don't seem to have been very active marketing the range post war. Possibly they thought it was too old fashioned. Eventually Johnsons of Hendon got the agency about the time of the P25 launch and the brand went from strength to strength in the UK. Their two biggest triumphs being the P8 8mm projector, launched in the mid 50s and the Mark-S 8mm sound projector launched mid 60s. Both of these masterpieces for their time and setting the standard for the others to follow.

Sadly Eumig were one of the first casualties when cine was replaced by home video in the early 80s. This was not helped by their tie up with Polaroid to produce the ill fated polarvision equipment. However none of their competitors fared much better - the market had simply disappeared.

[ricard]

This particular machine seems to have lived its life in Denmark, judging by the case it was in and the plugs on the associated power cables.

I definitely agree with your findings, as to me it seems unlikely that the machine is as old as from the 1950s, while at the same time looking closely at mid-1950s versions such as the P25 there are definitely differences which make this one look older.

I'm curious though, how would you tell an early 1950's machine like this one from a prewar edition?

[wd40addict]

Unless the finish was changed you're down to things like the capacitor date codes. New models every year did not arrive in cine until much later with the Japanese.

There were no great technology leaps in silent projectors until the arrival of the low voltage lamps so things stayed much the same.

[trobbins]

Back in the early years (circa 1930), Philips used to mark their chokes with the number of turns (eg. 7000 W), as well as on the schematic. No mention of the inductance value. The parts list showed the choke turns as well as core details. A dutch friend helped me decipher that quirk - the key being that W stands for windingen (ie. turns).