Ah, that's the entire point of the 50 ohm resistor: it cannot supply enough current to set off a detonator or squib while the Wheatstone Bridge is sensitive enough to measure the resistance of the complete firing circuit (including detonator, etc.).
There's currently a "Continuity Tester" on eBay as well, which uses an "800" (3-volt lantern battery) and a buzzer and is only for testing the cabling - it specifically warns against testing demolition circuits because they WILL go off.
(I have a detonator/squib tester that is a piezo-electric crystal and a neon - the energy supplied is nowhere near enough to warm up the bridge wire in the item being tested - though I suppose if that was open circuit inside the pellet the voltage might do it (instructions warn to put the item under test in a suitable container for safety).)
"Notes On Electricity (date)" is your friend, originally a small pocket book (1915), later a series of pamphlets "Notes on Electricity as applied to Service Apparatus" (I think), which covers telephones, gunsight illuminating gear, firing pistols for field artillery, and demolition kit. I have some of the books and pamphlets.
Best regards,
Chris.
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