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FOURTH DECADE  
7000-hp blooming mill), automatic control gear for compressors, coke oven equipment, and multi-motor contactor boards for coal preparation plants such as that for the Comrie colliery, the most up-to-date in the country. By 1930 control gear for the vital auxiliaries in power stations was being provided with latched-in contactors to ensure continuity of operation.

Liquid controllers were developed in 1933 for controlling 1500-hp and 2000-hp winders and were the largest that had been built by the Company. They catered for reverse current braking and included an automatic acceleration device.

The development of gate-end boxes, one of which received an early 'Buxton certificate' (August 19, 1932) from the Mines Department testing station, came to a head in 1937 with the MU 81, now a household word in mining circles. Equipments for coal cutting, conveying, drilling, lighting and section feeders completed the range for service below ground. Flameproof haulage control gear, including a cam-contactor controller, was introduced in 1936.

In 1937 the Company introduced a system of 'unified boiler control' for use in power houses. In this a d.c. supply to the armatures of the boiler auxiliary motors is varied by means of a single rheostat, thus controlling the motor speeds. As the boiler is operated with its dampers wide open the gas system resistance is a minimum, and the least possible power is taken by the auxiliaries. Moreover the motor fields are provided with tertiary windings arranged so that ideal combustion conditions are maintained over the whole range of boiler load. This most efficient and flexible of controls has so far been applied to seventy-five boilers, representing a total evaporation of 2500 tons of steam per hour and ranging from small industrial boilers to the largest high pressure boilers in operation in this country and overseas.

TRACTION
Investigations into commutation, electrical and magnetic losses, ventilation, and mechanical problems produced traction motors of lighter and more efficient design. Traction gearing, transformer 'metadynes', auxiliary motor-generators, and generators for Diesel-electric locomotives became established products of the Sheffield works.

The metadyne, in which the Company became interested in 1932, is one of a family of cross-field d.c. machines designed to utilize armature reaction. In the original dynamo invented in 1904 by Rosenberg, who became the Company's chief electrical engineer, the cross field was obtained by short-circuiting an additional set of brushes, but it could also have been produced by connecting them to an external supply. This was the method employed by J. M. Pestarini, later Professor at the Institute Electrotechnico Nazionale Galileo Ferraris at Turin, in his transformer metadyne.