In 1946, Ashby described the design of the units thus " Its principle is that it uses multiple coils in a milliammeter & uses the needle movement to dip in a trough carrying a current, so getting a potential which goes to the grid of a valve, the anode of which provides an output current . " It was the realization of what he had described in 1946 as an " Isomorphism making machine ".
2.
These systems are identifiable by the " press and hold for x minutes " in the pilot lighting instructions . ( The holding current requirement of such a valve is much less than a bigger solenoid designed for pulling the valve in from a closed position would require . ) Special test sets are made to confirm the valve let-go and holding currents, because an ordinary milliammeter cannot be used as it introduces more resistance than the gas valve coil.
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