Even more test meters


A trio of meters

 

 

 

 The origin of these three meters is not known. "Philip Harris Ltd", (whom I'm reliably informed, by Peter Raven, was a Birmingham-based supplier of scientific instruments and apparatus to schools and laboratories up to at least the mid 50s) is stamped on the base and "British Make" is printed on the faces of the two on the wooden plinths, whilst "British Made" is printed on the smaller test meter. All meters are finished in bright nickel plating and all three are completely undamped and are of the "Moving Iron" type which are inefficient but can measure AC as well as DC without the need for a rectifier.

I don't think the wooden stands and the meters started life together as, if you look carefully, you can see a shadow of a larger meter around the peripheries. There are also screw holes in the wood for slightly larger meters. The two meters with the side connections look 1925 or earlier whilst the one with the cord looks typical of the meters used by constructors of radios in the 20s and 30s. Between 1925 and 1934 valve heaters were lit by 2 volts DC or 4 volts AC, thus the meter would be just right for this purpose. Later 6.3 volts was used and this would clearly be too high for the meter. Earlier you would have not encountered many filament voltages higher than 2.

Big shunts

 

 

 

 

 This heavy lump of copper, top left/right, which is about ten inches overall, is for measuring very high currents. Between the two copper blocks are a number of metal strips designed to drop 50millivolts at 500Amps. The resistance between the two small terminals is therefore one tenth of a milliohm or one ten-thousandth of an ohm. This enables a small meter having a full scale deflection of 50millivolts, connected across the terminals to be calibrated at 500Amps fsd.

Surprisingly, even this very low resistance will dissipate 25 watts of heat when carrying the current indicated.

Underneath stamped on the underside is the letter "W", the trade mark of Westinghouse.

The lower two are rated at 250Amps and 300Amps respectively and require a 75mVolt fsd meter in order to provide full scale readings of these currents

Below, another which I unearthed whilst tidying up. I think this one belongs to a test meter I have?

 A pair of Galvanometers

 I have two of these French galvanometers.

These are examples of a mirror galvanometer, a very sensitive device for use with something like a Wheatstone Bridge.

The greater the sensitivity of current measurement, the greater the accuracy of measurement of resistance for example.

I bought these from a chap in France, where I imagine they were used in a Physics Laboratory. This particular design was invented by a chap named D'Arsonval.

The mirrors are presently detached but normally would be suspended so that they are visible through the front window. A light source can be directed at the mirror, which reflects a spot onto a remote scale. Inside the case is a horse-shoe magnet and the flux from this reacts with the field developed by current passing through a coil mounted with the mirror. The result of this interaction is a mirror deflection.

 

 
 

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