Moorhead Laboratories of San Francisco, California, supplied both R-valves and B-valves to the British government during the First World War, to meet the huge military demand. According to Otis Moorhead, the R-valve was designed to the following specification.
Rectangular sheet of pure nickel 31 mm × 15.2 mm, rolled into a cylinder of 10 mm external diameter, the thickness being 0.2 mm.
Nickel wire 0.25 mm diameter and165 mm long, bent into a helix of eleven turns. The helix pitch to be 1.5 mm and the external diameter, 4.25 mm.
The grid to be set so that nine of its turns are within the anode cylinder and one turn projecting at each end.
Pure drawn tungsten wire of 0.061 mm diameter and 23 mm long.
Not to exceed 54.76 mm external diameter and the length from the remote end of the pin on the base to the top, 111.91 mm.
Diameter 3.171 ± 0.075 mm. Length of pin projection from base 17.462 mm.
The specification did not indicate whether the electrode assembly should be mounted vertically or horizontally. Contrary to European practice at the time, Moorhead chose to mount the assembly vertically as here.