[28 
CARGO HANDLING AT PORTS 
[n addition to handling ore (which, in order to be suitable 
should contain, at least, 60 per cent. of iron), iron scrap, 
and steel rails can be conveniently dealt with. 
The method of working is quite simple. The magnet is 
suspended from the jib of an ordinary crane, either steam or 
slectric. In the case of an electric crane, the only additional 
fitting required is an extra controller for exciting the magnet 
when in contact with the material to be handled, and 
releasing it, as necessary. Magnets are only suitable for use 
with direct current, and where alternating current is the 
source of supply, some means of conversion must be provided, 
such as a motor generator. A voltage of 220 is usually 
considered the maximum, though magnets have been 
adapted to higher voltages. In the case of a steam crane, 
it is necessary to install a steam-driven generator to supply 
current, which will preferably consist of a turbine set, so as 
to occupy as little floor space as possible. 
The possibility of failure in the current, with the conse- 
quent release of the load while suspended, is a contingency 
which, while sufficiently remote, at any rate suggests itself 
as a prominent consideration, and, to get over this difficulty, 
a special safety device has been provided in certain cases. 
This consists of a set of accumulators connected up to two 
automatic switches carried on the top of the magnet frame. 
Another switch enables the driver to put the accumulators 
on charge whenever the magnet makes a lift, and to dis- 
connect again when the load is deposited. If the main 
~urrent should fail for any reason, the accumulators at once 
and automatically provide current sufficient for the magnet 
‘0 retain its hold on the magnetised load. 
There is one rather serious objection to the use of the 
magnet crane at the quayside which practically limits its 
ase to barges and other small craft, viz., the detrimental 
affect which it has on ships’ compasses. It becomes neces- 
sary to have the compasses re-adjusted after a magnet has 
been used in the vicinity. For this reason, navigation 
officers have a very strong antipathy to the appliance, and 
it ie difficult to see how this can be overcome. It may be