182 SECTIONAL ADDRESSES. 
labour of re-carboning have led to its abandonment in favour of the less 
efficient filament lamp. 
Before leaving the subject of electric lighting I would point out that it 
is remarkable that the first great application of electric power should have 
been for the production of electric light, since it is probably the least 
efficient of allits applications. The overall efficiency of a small power station 
supplying a lighting load and having therefore a very poor load factor 
would not be greater than about 6 per cent. from coal to switchboard, 
the steam-engine being, of course, the principal offender. Of the total 
power supplied to and radiated from a carbon-filament lamp not more than 
about 2 per cent. was radiated as light, so that the overall efficiency from 
coal to light was 2 per cent. of 6 per cent., which means that of every ton 
of coal burned at the power station with the object of producing light all 
but about 3 lb. was lost as heat at various stages of the transformation. 
Even now, with up-to-date steam plant and gas-filled lamps, the overall 
efficiency from coal to light is not equivalent to more than 40 to 60 lb. of 
coal out of each ton. The electrical engineer may derive a little comfort 
from the knowledge that the purely electrical links are the most efficient . 
in the chain. 
Whilst on the subject of efficiency I might point out that the difference 
between the prices at which coal and electrical energy can be purchased 
by the ordinary citizen corresponds to the losses incurred in the power 
station; that is to say that the cost of the generation and distribution of 
the electrical energy is covered by the better terms on which the power 
station can obtain fuel. In Glasgow the writer pays 51. per ton for anthra- 
cite to burn in a slow-combustion stove; taking the calorific value of anthra- 
cite at 9,000 kilowatt-hours per ton, which is equivalent to 14,000 British 
thermal units per lb., this works out at 74 kilowatt-hours for a penny. 
For electrical energy for heating and cooking purposes the writer pays a 
penny per kilowatt-hour. This ratio of 1 to 73 will correspond fairly 
closely to the overall efficiency of the power station. In view of the high 
efficiency and convenience of slow-combustion stoves, it is evident that 
electric heating cannot be expected to compete with them for continuous 
operation ; for intermittent heating the question is very different. 
Returning from this digression to the development of the direct-current 
dynamo, it may be noted that the drum armature now almost exclusively 
employed was invented in 1872 by von Hefner Alteneck, and gradually . 
displaced the ring armature of Pacinotti and Gramme. Although Pacinotti’s 
original ring armature was slotted, smooth armatures were preferred for 
many years, until the mechanical superiority of the slotted armature caused 
the disappearance of the smooth core with its wooden driving pegs which 
were employed to transmit the turning moment from the conductors to 
the core. The commutator and brushes were a great source of trouble, 
but by the gradual elimination of unsuitable material and by better design 
and methods of manufacture the commutator has been made a most reliable 
piece of apparatus. The difficulties of commutation, and especially the 
need of continual adjustment of the brush position, were largely overcome 
by the invention of the carbon brush by Professor George Forbes in 1885. 
It should be pointed out that the commutating poles, which have come 
into use so much in recent years, were originally suggested in 1884, and 
are therefore older than the carbon brush. 
