The reverse of the card reads: 'Grid Bias Leads. Two lengths of coloured insulated wire, one 6 inches long (black), and one 9 inches long (red) are fixed to black and red "wander" plugs (Lisenin Positive Grip). The other end is joined to the terminal which is marked "Grid Bias". The red wire is similarly screwed under the filament terminal of the first valve holder to which the earth terminal is connected.'
© Science Museum / Science & Society Picture Library