I received a very old IG-18 the other day. The lamp itself is marked SYL 90V. An online cross-ref page listed a Sylvania 90MB, a T 2-1/2 bulb in a bayonet base. It's rated at 90 volts and 30 milliamps, for a hot resistance of 3k ohms. I measure the cold resistance at 280 ohms out of circuit. During the measurement, the HP 3458's 4-wire ohms current increased the resistance 10 ohms. Dick Moore