Pictured here are a Ruud gas sidearm, left, and a Wesix electric sidearm. At one time there were more than 150 water heater manufacturers and many made sidearm heaters. There were lots of permutations.
The Ruud has two independent coils so either one could be replaced if needed. Later versions had insulated sheet metal casings for lower heat loss and more durability. Automatic control was also done so that they needed no more attention than a modern heater.
The Wesix heater has a 4,500-watt element which is the same power as most modern elements. The interesting thing is it is more than four times bigger. With so much surface area, it would not produce any quantity of sediment or scale. It was made in 1923 and we found only a bit of dust in the unit, no scale at all.
The temperature control is unusual also. An alcohol-filled chamber expands when it reaches 150 degrees. This flips off the power to the heater. There is no other control other than an on-off switch. 150 degrees was normal in 1923!