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  • Author Author: neilk
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Vintage milliAmmeter - ex school/college lab demonstration

neilk

I first spotted this large milliAmmeter in the window of an Antiques Shop in Castle Donnington, Derbyshire, a few weeks ago. Unfortunately the shop was closed, but I decided to go back for a closer look when the shop was open.

 

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I found the shop open last Saturday and had a good look at the instrument. As can be seen  against the e14 little bue man, it's quite large, typical of the instruments I can remember being used on the teacher's/lecturer's bench many years ago! I suspect it dates from the 1960s or 70s.

 

When I released the pointer lock, the movement swung freely, so I was hopeful that there was no physical damage. I couldn't test it electrically, but I decided to take a punt, and buy it, if the price was right. Even if the movement was burnt out, it would look attractive on display somewhere near my desk/workspace.

 

The dealer was asking £40 GBP; I offered him £30 GBP, which he accepted without much hesitation! It fitted into a large carrier bag I happened to have with me and off we went. As we walked away from the shop I commented to my wife "That was easy. I should have offered him £25 GBP!". She replied "Don't be so tight!" He won't have been able to open much since March and probably needs the money!"

 

The milliAmmeter was made by the WHITE Electrical Instrument Company, founded in 1911 by Herbert Brandon White. The company has now ceased trading, but there is quite a bit about the history of the company on the internet, with photographs of workers and workshops. I also found some listings of various White instruments being sold at  auction - one similar to mine which sold for £50 GBP earlier this year. image

 

 

Here are a some close-ups of the movement - taken through the glass:

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I have now tested the instrument and it is in full working order and surprisingly accurate when compared with my SIGNSTEK DMM.

 

I was very careful testing the milliAmmeter - the last thing I wanted to do was to burn out the movement coil!

 

Below are the circuits I used for testing: the top schematic shows current checking with the DMM, using 3 different resistors, the bottom schematic shows the milliAmmeter in circuit as well.

 

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I checked the nominal value of each resistor with the DMM, before using them - I have great difficulty reading the color codes on some types of resistor! I was only concerned with an approximately correct value; I didn't spend time selecting resistors for precision.

 

I started with the 220 kOhm resistor for safety's sake:

 

220 kOhm resistor: DMM reads 14uA - needle on milliAmmeter barely moves.

 

22 kOhm resistor:   DMM reads 129 uA - milliAmmeter reads just over 0.1mA.

 

2.2 kOhm resistor:  DMM reads 1.37 mA - milliAmmeter reads 1.38 mA.

 

I'm very happy with the results and very very happy with my purchase!

 

Apart from the decorative possibilities, I'm now looking for some ideas of what to do with it!

  • in reply to shabaz

    Hi shabaz,

     

    When I first saw it in the shop, it was high up on a shelf at the back of the window and I couldn't see it very clearly, but I had a feeling about it! When I got to look at it properly some weeks later, I was pretty sure I had to buy it! The Science Museum group has some similar instruments from the same maker in their  collections. However, they have no internal additional coil and the meter movement connections appear to be  brought out directly to the terminals. One instrument is shown with a Voltmeter scale and the other is shown with an Ammeter scale. The scales are removeable and I am assuming that these instruments were supplied as a "kit" with a set of scales, shunts and series resistors, but I have no evidence to support this!

     

    I have had a good look at the movement and coil you refer to. Without removing the glass to get a closer look and, dangerously?!, checking continuity, I am pretty sure that the coil is connected directly across the meter movement and, therefore, a classic shunt.

     

    Incidentally, during the tests outlined above, I measured the voltage drop across the instrument at about 40 mV when it was reading  about 1.38 mA. That gives a combined resistance for the meter movement and the shunt of about 29 Ohms. However, I have no way of knowing what the split is, without dismantling the instrument, which I am not about to do!

     

    Neil

  • Hi Neil,

     

    It's a nice-looking meter! It looks good enough to be on display in a museum : )

    I wonder what the coil on the right side is part of.. maybe for damping? Or some additional experiment, say shorting the inputs, and then moving a magnet, although it seems to be in a physically hard to get to location to do that.

  • in reply to dubbie

    Dubbie

     

    I have close relatives around the area,

     

    Neil

  • in reply to neilk

    Neil,

     

    It was some years ago now so I have no more information on the large meter.

     

    I left Caste Donington 45 years ago. A long time. You must still live locally then.

     

    Dubbie

  • in reply to dubbie

    Dubbie, (I'm Neil, BTW)

     

    Would be interested to see a photo of the one you have seen, if you can get one, and the price, of course.

     

    Coincidence re: Castle Donnigton; when did you move away?

     

    There is still a fish and chip shop, but I can't comment on it. I have eaten in a couple of places in the Village - not bad.

     

    Neil