THE PROGRESS OF THE ELECTRIC LIGHT 13 



candle power has been taken as the basis of comparison. The 

 mean spherical candle power is the candle power of a source 

 of light radiating with equal intensity in all directions the same 

 total amount of light as the source under consideration. No 

 other candle power is of any value in a comparison such as 

 has been made. It is important to remember this when figures 

 for the candle power, efficiency and cost of lighting are con- 

 sidered. Figures double those which have been given for 

 arc lamps will frequently be found : these represent the mean 

 lower hemispherical candle power and are double those for 

 the mean spherical, because most arc lamps give nearly all 

 their light in the lower hemisphere. Their use is justified 

 on the plea that it is only the light in the lower hemisphere 

 that is useful : although this may justify the practice to a certain 

 extent when the comparison is only between one type of arc 

 lamp and another, the justification falls to the ground when 

 they are compared with incandescent lamps or gas lamps. The 

 mean spherical candle power is the only measure of the total 

 light given by the lamp and therefore the only true basis for 

 comparison. 



No reference has been made to one or two electric lamps 

 which have barely been advanced beyond the experimental 

 stage, such as the magnetite arc, the mercury vapour lamp and 

 the Moore vapour lamp. The magnetite arc uses electrodes of 

 a mixture of magnetite, titanium oxide and chromium oxide 

 forced by pressure into an iron containing tube. A white flame 

 arc is produced with a consumption of about 07 watt per 

 candle : as a compensation for the low efficiency compared with 

 that of the flame arc the electrodes have a long life— from 100 to 

 150 hours. The lamp has been developed in America and is in 

 use there to some considerable extent. 



The mercury vapour lamp consists of an arc between 

 electrodes of mercury enclosed in a tube of glass or quartz. 

 The efficiency is high, the consumption being from 0*3 to 0*4 

 watt per candle but the light is almost entirely devoid of red 

 rays and consequently of a colour that makes the lamp im- 

 practicable in all but a very limited number of cases. Some 

 slight improvement has been effected by using quartz tubes 

 which enable the arc to be run at a higher current density but 

 there is still a long way to travel before a satisfactory result 

 is obtained. 



