Dialogue:

P. For what purposes are electric motors used?

T. Electric motors are used for practically all purposes in nearly every section of a mine.

P. What types of motors do you know?

T. Electric motors may be operated either by direct current or alternating current.

P. What type of motors is most widely used nowadays?

T. Great attention is devoted principally to the widely used three-phase alternating-current machines.

P. Into what classes can these machines be divided?

T. Such machines can be divided into two chief classes, viz, the squirrel-cage motor and the slip-ring motor.

Coal in early britain

There is no evidence that coal was known before Roman times but there is little doubt that it was used to some ex­tent by the Roman colonists. Thus coal cinders have been found in Roman remains on the sites of forts. Romans used coal for smiths' work, for lime burning and for heating purpose. Its use, however, was very limited, and no Roman remains have been found in old coal workings to indicate that it was obtained from underground mining operations. Even where Roman camps were established not far from the exposed coal­fields, little use of coal was made.

When the Romans left the country, the use of coal as a fuel ceased for a long time and centuries elapsed before the birth of the coal-mining industry as we know it today.

The native population and the Saxon and Danish invaders of Britain found timber sufficient for their needs, and they did not make any use of coal.

Later Normans recognized the value of coal as a fuel. In some areas of England the Coal Measures extend under the sea, and as a result of erosion, coal was (and still is in some areas) collected and sold for use as fuel in place of wood, and is known as "sea coal" to distinguish it from charcoal.

As time went on, not only did the demand for coal for home consumption increase, but foreign markets opened, and as early as 1325 coal was being exported from Newcastle to France.