Alum Rock designs its Tranformers for performance, appearance, reliability, minimal heat dissipation, and minimal stray field or suseptibility to stray fields. Alum Rock places little emphasis on minimizing size, weight, or cost. Units which carry direct current generally have internally gapped cores. All transformers and inductors are vacuum-impregnated. High voltage transformers such as output transformers go through three separate potting cycles. A Biddle corona testing facility is used to measure the corona current for all high voltage units. Output transformers are tested for corona current at 1,200 Vrms + 2,000 VDC.