As the temperature of the metal rises the mercury in the bulb expands causing the coiled spring to uncoil slightly, driving the contact lever against the disk that shuts off the current, allowing the switch to fall open and disconnecting the circuit through the crucible heaters.
As the temperature of the metal falls, the mercury in the bulb contracts allowing the coiled spring to force the contact lever against the disk that turns the current on by connecting the switch to the main line. The variation in the temperature is held within 15 degrees of the average working temperature.
The temperature control is normally set for a maximum of 550 degrees and a minimum of 535 degrees, that is, with normal operation the temperature of the metal will always be between these limits. This is found to give the best all around casting results for average metal. In case it is desired at any time to change the operating temperature, this can be done by turning adjusting screw right-handed for hotter metal, and left-handed for cooler metal. The head of this adjusting screw projects through the right-hand side of the dynamic thermometer cover.
Do not change the position of the control disks, unless badly pitted, as their relative distance is determined at the factory, and very seldom need to be changed.
A good thermometer should always be used when adjusting the screw for proper heat. Guessing will not get the proper results.
With ordinary work, and after the metal is at operating temperature, the current will be on and the crucible heaters generating heat about three minutes, then off and not generating heat about twelve minutes, and will repeat this cycle as long as snap switch is left in the “on” position.
Throat and Mouthpiece Heaters
The mouth and throat heaters are clamped in close contact with the outside of the crucible throat to keep the metal at the operating temperature while being pumped from the crucible to the mold. The throat heater extends the full length of the pot throat and is held in close contact with it. The mouth heater is clamped tightly to the pot mouth. Both heaters are surrounded with heat insulating material and are held in close contact with the pot throat and mouth by a U-shaped clamp under the throat unit and a plate over the mouth unit, drawn tight by two nuts.
The mouth and throat heaters are always connected in “series,” one set being used for 100-volt to 125-volt and another set for 200-volt to 250-volt.
The heat for the throat and mouth units are controlled by a manually controlled rheostat moving from the left to the right, and fastened to the front of the control panel under and at the rear of the keyboard.