GTS Dave said

You use the existing voltage as a source. You put the decade box inline with the wire from the existing sensor. All it does is add resistance to the current resistance (you knew that..)
Here's one for even cheaper.. Elenco Resistor Substitution Box - RS-400 756619000299 | eBay

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GTS Dave said

You can get decade boxes with it's own power source, they just cost more..

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