of a general supply of electricity to house-holders. But lamps for 440 volts are not obtainable. To use a supply at 440 volts with the lamps at present on the market, we should have to use two in series connection, that is to say, always burn lamps in pairs. This would be an intolerable restriction to which no householder would submit. Now suppose for a moment that we do not put two neighbouring lamps in series, but two neighbouring houses. This means that a tapping from the positive main only is taken to supply house No. 20, and a tapping from the negative main only to supply house No. 21, the circuit being completed by a wire taken from the lamps of No. 20 to those of No. 21. If both householders were to agree that they would at all times burn exactly the same number of lamps, we should have electrically the same condition as in the previous case, where we arranged the lamps in one house in pairs.
This arrangement would, however, be still more intolerable than the previous one. Now suppose that we put all the houses of even numbers on the positive main and all the houses of odd numbers on the negative; further, that all the connecting wires between the houses are replaced by a third main, then
