Precision Apparatus Dynamic Electronometer

[David Pannell]

I have had, for quite some considerable length of time, a model 920 in pretty good condition and is working well.

However, the 300 ohm variable rheostat for adjusting the 120 volt mains voltage for optimum setting, and also for zeroing the high resistance ohms range; has always run very hot.

As the instrument is housed in a wooden case, I would be happier if I could fit a higher wattage rheostat. The problem is that there is no rating, only the resistance marked on it. The instrument consumes approx. 0.4 amps at full load, which is about 48 watts, and the manual states that a 45 watt transformer would be adequate for running the instrument from a 240 volt supply. Even the schematic diagram and parts list only give the resistance and not the wattage of the rheostat.

The rheostat is physically quite small with fairly fine wire, and looks nothing like a 50 or so watt rating.

Therefore, if anyone has any experience of this instrument, or knows the rating of the rheostat, I should be most grateful.

With many thanks and best to all,

Just get a bigger one, by the laws of physics (approx) it will have less temperature per unit area.

[David Pannell]

Thought of that - thing is there isn't much room, so I really need to know wattage in order to be sure it IS bigger - electrically, that is.

But thanks for the thought. It would be the logical answer if it were that simple.

Cheers.

[Lucien Nunes]

Fitting a rheostat with a higher electrical rating won't change anything unless it is bigger. The amount of heat dissipated depends only on the current flowing and the resistance value, neither of which you can change. A more modern rheostat of similar size will have a higher rating only because the rheostat itself can withstand being run at a higher temperature. So as MM says, the only way you can improve things is to get a physically bigger one which will dissipate the same amount of heat with a lower surface temperature.

Presuming you have a good range of adjustment on the rheostat setting, how about using a transformer or variac to lower the input voltage slightly, so that the rheostat can be adjusted until much of it is out of circuit? That will reduce the total heat generated inside the instrument which changing the rheostat would not.

Lucien

[David Pannell]

Good thinking, Lucien. Thanks

If you have a bit of panel space try a 'heatsink' to spread it a bit. i.e. a bit of sheet ali or copper to distribute the therms.

[David Pannell]

Another good plan! Panel space is somewhat limited, but the heat sink could be bent and dressed to suit.

Cheers.