The reverse of the card reads: 'Inside of Cabinet. The cabinet may be as simple or elaborate as taste and pocket decree. Oak or mahogany is suitable, and the material should be 1/4 inch thick. The 5 ft length of 7 1/2 in wood is sufficient to make a cabinet with plain butt joints fixed with No 1 screws. The illustration shows how the grid bias battery (9 volt) is secured to the back of the cabinet. Two large washers being used to enable the 1 1/3 in No 6 screws to hold the battery in position.'
© Science Museum / Science & Society Picture Library