School immersion heaters are often small metal tubes with flying leads which enable connection to a labpack. They are usually low voltage (12V) and should only be powered using the recommended voltage. This voltage is usually written on the heater or in a manual/instruction booklet supplied with the device.
Lab packs with ‘dial’ type voltage adjustment should be locked down if possible to the correct voltage when immersion heaters are being used. Higher voltages may irreversibly damage the heating element inside each heater.
Immersion heaters are often used in conjunction with calorimeter blocks in experiments to determine specific heat capacity and also sometimes used to show energy transfer (electricity to heat).