The electrically oxidised eureka wire is wound on perfectly round 43mm dia porcelain pipe according to length required. The vulcanite/Aluminium hexagonal/round supports light the pipe on each side with a rod and nuts. The guide rod carrying the contact block is lightened with screws to the projection of the supports and carry the third terminal which permits the instrument to be used in any step. The contact block carries smooth phosphor bronze brush contacts. The instrument is perfectly insulated.