Electricity lowing through the elements causes them to heat up. The rate of heating is controlled by switching the electricity supply on and off - this may be every 30 seconds where mechanical relays are used, or on a cycle by cycle basis with thyristors.
When the element is hot, it will dissipate heat in three ways:
- Conduction: heating up other solids that the element is in contact with, such as the element holders. Heat will flow from hot to cold, but as the element holders are made of an insulating material this is a slow process
- Convection: heating up air in contact with the elements. As it heats up, the air expands and becomes more buoyant, and so rises. This causes air currents within the kiln, and the heated air heats cooler objects it comes into contact with. Also, the buoyancy of the heated air means it will tend to rise to the top of the kiln, resulting in the bottom being cooler
- Radiation: this is electro-magnetic radiation that doesn't need a "carrier" like air, and travels in straight lines from the element until it hits something. It is also responsible for things glowing as they heat up. Radiation dominates at higher temperatures, as the radiated energy increases with the 4th power, so double the temperature and the radiated energy goes up by 16.
The kiln walls, shelves and pots are all heated up by these three mechanisms, and will in turn heat other objects by the same mechanisms. In time, this evens out the heat distribution in the kiln.