[volt-nuts] Ayrton Shunt does double duty.

Bruce Griffiths bruce.griffiths at xtra.co.nz
Fri May 4 17:36:38 EDT 2018

The volt box is likely to be a voltage divider that was used to divide a voltage to be measured down to a value within the measurement range of a L & N potentiometer (not the 3 terminal component but the type that sets up the current in a series string of resistors to a known vale by comparing the voltage across a subset with the voltage of a standard cell. The unknown voltage can then be measured by comparing it against the voltage drop across a adjustable subset of the series resistor string).
They are mentioned in various NBS (NIST) publications of the era  as well as in various texts on dc electrical measurements. They were in common use up till around the 1980's.

> On 05 May 2018 at 01:55 geoelectronics at rallstech.net wrote:
> Hello Dave. 
> So far I have identified 3 different Leeds & Northrup "Boxes".
> Originally my intention was to re-purpose the box, panel connectors and
> switch for a project to make my own precision resistor banks. 
> All use different switches and vastly different resistors to achieve
> their original purpose. 
> The subject of the first one is the "Ayrton (wired) Shunt Box". It was
> used with a galvanometer as a sort of attenuator (current divider),
> uniquely wired in the Ayrton fashion to insure the galvanometer was
> always connected to a shunt resistor, even during switching. Think of
> the scheme as a stepped potentiometer, the meter was between the two
> extremes, while the source went to the wiper. Just backwards from a
> Voltage divider. 
> Next is the "Shunt Box". It's resistors are copper straps, the lowest
> value is made from a copper sheet maybe 5 inches wide, bent into a
> corrugated shape to fit inside the box. Remarkable construction, should
> be quite stable as a milliOhm standard. 
> The last one is called "Volt Box". Its resistors are wire wound on
> ceramic forms, obviously with great care. The switch is protected and
> possibly gold plated contacts. Multiple series connected resistors are
> used to provide the correct values plus spread the current (heat load)
> around, and they are spaced far apart and with air space around each
> one. So far no clue as to its original function but of course I have
> traced the schematic. 
> Very little is freely available on the 'net about them nor a period
> catalog so far. 
> George Dowell
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