'How to build a two valve set', No 20, Godfrey Philips cigarette card, 1925.
© Science Museum / Science & Society Picture Library
The reverse of the card reads: 'Battery Connections (Contd). The fire wires are now twisted into a cable leaving 8-inch ends loose for connections to the terminals in the set. 4 inches may be cut off the black and brown wires, the bared ends of both these wires are clamped under the filament terminal of the second valve, as shown in the picture. The three remaining ends are bared and are connected as follows: Red to the spare terminal on the filament rheostat, Green to the + H T terminal on the transformer. Gold to the top earphone terminal.'