Impossible circuit

PabloPicasso

PabloPicasso

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The steaming light stopped working. Went up and changed the bulb. The old bulb is the festoon type with dimpled ends and the filament looked intact. Tested it by touching one end on the + batt terminal and connecting other end to neg via a circuit tester. The tester bulbs lit up but the filament did not! Can any one explain how his cold happen? In case you don't believe me I'veosted a pic
 
Try one end of the bulb to the battery and the other to the negative via a piece of wire (you migh need three hands?). Might be that the circuit tester has a diode so that it only works one way round??? Andrew
 
If the tester is an incandescent bulb it will be tiny so high resistance low current. You have connected the lamp being tested in series with the tester. The steaming light is probably 18 watts so 1.5 amp operating current or 8 ohms. In fact much lower when cold.
The tester bulb is probably about 50 ma bulb so .6 watt or 240 ohms. The power to light the lamps is proportioned to the resistance in the series circuit. So the tester gets most of the power having the higher resistance. Or put another way to the tester the steaming bulb appears to be a short circuit and is relative to the tester bulb. Hence tester lights but large bulb does not. Or if it does it will be very dim. Of course as said if it has an LED in it then even less current through tester to light it. good luck olewill
 
The bulb is OK. What was stopping it working was probably a corroded connection and (assuming it now works) putting the new bulb in has cleaned it up a bit. If you want to test the bulb properly just put it across the battery without the tester.
 
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