railways at home and abroad. One of the largest installations in this country was carried out at Bristol in 1936; this was an all-electric interlocking equipment having three power frames with a total of 728 lever spaces. Paddington station had been completely resignalled with similar equipment three years earlier. Automatic signalling was provided between Euston and Watford in 1932 and between Shenfield and Southend in 1937. (Power signalling was installed on the steam lines between Camden and Sudbury in 1941.)

WELDING
Electric welding equipment, both arc and resistance, made rapid progress. About 1930 the growing popularity of bare wire electrodes brought up the question of arc stability, particularly on the lower current range, and in the following year 200-A and 300-A welding sets were developed, giving a much steeper volt/ampere characteristic. This was obtained by using a diverter resistance connected across the series field winding, fine adjustments being provided by shunt control. In 1931 also there was introduced a single-operator a.c. arc welder consisting of an air cooled transformer to step down the supply voltage to 85 V and a tapped choke coil, built into the transformer, to give about 25 V at the welding arc at all currents.

In 1933 further investigations into arc stability led to the development of new generators having still better dynamic characteristics, and two years later the Paradyne single-operator welding set was introduced; this design had no exciter, therefore being lighter and requiring less maintenance, and the field poles and windings had been redesigned to improve the welding characteristics. A new type of a.c. arc welding equipment was brought out in 1937—the Thermae single-operator welding transformer; this was an air cooled unit in which infinite variation of the welding current was obtained by adjusting the transformer core.

Automatic arc welding was tackled from about 1929. The machines developed were installed first in the Company's tank shop for welding large switch tanks and a year or two later in other works for boilers, torque tubes and pipes. Equipment for atomic hydrogen welding was also developed, and by 1933 both hand welding apparatus and an automatic set for transformer radiator tubes were in use in the works. In 1937 an automatic equipment was made for welding Bren gun magazines; further sets were sent to Canada.

Welding electrodes were made from 1932, braided electrodes being produced at a rate of about 30 feet a minute. Five years later the use of the extrusion process enabled the speed of production to be stepped up to a maximum of 300 ft a minute.

Resistance welding machines were also developed, starting in 1935 with a 300-kVA spot welder for aluminium and aluminium alloys in the aircraft industry. This machine, the first of its kind in the country, was designed to give the high currents and extremely short welding times required; it was provided with fully synchronous control. Aircraft spot welders were developed in the next year or two in 390 and 650 kVA sizes, and also a smaller 350-kVA machine provided with high voltage thyratron control.