'How to build a two valve set', No 11, Godfrey Philips cigarette card, 1925.
© Science Museum / Science & Society Picture Library
The reverse of the card reads: 'Wiring the filament circuit. A filament rheostat is fitted in the remaining hole in the panel. This instrument shown is a "Microstat", but any make of panel mounting will do. The resistance will depend upon the values used. Advice on this point will be given at the place of purchase. 5 ohms is suitable for the valves recommended. The filament terminals of the valve holders are connected in paralell, and the side nearer to the panel is wired either one of the terminals on the rheostat.'