The Standards of this section apply to acceptance tests for all types of metal tank mercury arc rectifiers used for power purposes together with essential auxiliaries. The essential auxiliaries are: (o) Evacuating system equipment. [b) Equipment for starting and maintaining the arc. (c) Temperature control and indicating equipment and other accessories. Transformers supplying power to rectifiers shall be considered part of the rectifier unit. Due to the action of the rectifier, the transformer windings, particularly those connected to the rectifier, carry currents which differ substantially from the usual sinusoidal form. This section, there fore, includes additional paragraphs, referring to the ^' transformer equipment of rectifiers to supplement the Standards for Transformers, Induction Regulators and Reactors (A^LE.E. Standard No.13 May 1930) on points where those standards are not sufficient or do not apply.