The tube is attached to a manifold which is then attached to a high-quality vacuum pump. The tube is then evacuated of air until it reaches a vacuum level of a few torr. The evacuation is paused, and a high current is forced through the low-pressure air in the tube via the electrodes (in a process known as "bombarding"). This current and voltage is far above the level that occurs in final operation of the tube. The current depends on the specific electrodes used and the diameter of the tube but is typically in the 150 mA to 1,500 mA range, starting low and increasing towards the end of the process to ensure that the electrodes are adequately heated without melting the glass tube. The bombarding current is provided by a large transformer with an open-circuit voltage of roughly 15,000VAC to 23,000VAC. The bombarding transformer acts as an adjustable constant current source, and the actual voltage during operation depends on the length and pressure of the tube. Typically the operator will maintain the pressure as high as the bombarder will allow to ensure maximum power dissipation and heating. Bombarding transformers may be specially made for this use, or may be repurposed electrical utility distribution transformers (the type seen mounted on utility poles) operated backwards to produce a high voltage output.