July 1, 1892.1 



KNOWLEDGE 



125 



ing a splendid and perfectly steady light. We used 

 platinum and alloys of platinum and iridium, and then tried 

 a multitude of forms of carhon, including burnt cane." 



These attempts, however, 

 were not successful in leading 

 to any practical application 

 on a large scale, and to Swan 

 and Edison belongs the credit 

 of first demonstrating the 

 capability of this method of 

 incandescence of carbon In 

 high vacua to give permanent 

 and satisfactory results in 

 actual practice. 



The following are the prin- 

 cipal details in the structure 

 of the glow lamps of Swan 

 and Edison, and the points 

 in which they differ. In the 

 recent types of Swan lamp an 

 organized material such as 

 crochet cotton is taken ; this 

 is treated with acid, whereby 

 the cotton fibres are reduced 

 to a gummy consistency 

 before they are carbonized. 

 Edison starts with bamboo 

 fibre, and has fixed on this 

 substance because it possesses 

 a definite structural form. 

 The shape of the carbon in Edison's lamp is a long arch, 

 or a carbon thread simply twisted back to run parallel to 

 itself. Swan's form ditfers in having in the course of the 

 carbon a spiral of one turn whose diameter is half 

 an inch. 



The thickness of Swan's carbon is about O-iJ.5 milli- 

 metres, /.('., -ji^ of an inch. Edison's 16-candle power 

 filament is oblong in cross section, the sides of the oblong 

 being 0-1 and 0-2 millimetres, i.e., -^i-^ and jl^ inches. 



The filaments of carbonized bamboo 

 in Edison's lamp are prepared thus : — 

 The canes got. from the plant, which 

 are tubular in form, are prepared first 

 of all by machinery devised for the 

 purpose, so that a large number of uni- 

 form pieces is rapidly obtained. The 

 canes are first of all sawn into suitable 

 lengths. Each of these pieces is then 

 split down its centre, and two half tubes 

 are thus got ; both of these are again 

 divided into three strips, the hard 

 outer rind containing silica is removed, 

 and the stripped pieces are shaved 

 down so as to produce a flat and 

 straight strip of uniform thickness 

 throughout its length ; these strips are 

 then cut so as to be of the same length. 

 After a set of similar fibres has been 

 thus prepared, they are placed in blocks 

 and cut so that their final shape is a 



Fig. 3. Edison's narrow strip of bamboo with enlarge- 



Lamp. ments at each end, by means of which 



they are joined to the comaecting wires. 

 The fibres are next bent into the required form, that of a loop 

 or horseshoe, and carbonized by being raised to a white heat 

 in muffles placed in a furnace. Next they are electro- 

 plated on to their platinum supports, this being done to 

 ensure a thoroughly good contact, and then placed in their 

 containing bulbs. These latter are repeatedly exhausted ' 



of air, whilst the fibre contained in them is again and 

 again raised to a very high temperature (by the passage of 

 an electric current) and allowed to cool down between its 

 successive heatings. By this process the air or any 

 occluded gas is got rid of, and besides this the fibres are 

 subjected to such a severe test that only those which are 

 quite sound can survive it, and in the end a fibre suited to 

 the purpose in view and capable of long endurance is 

 obtamed. 



The ejiicienvij of dift'erent lamps is compared by finding 

 the quantity of energy used up per candle pow-er produced. 

 This energy, which is consumed in the lamp, is equal to 

 the difference of potential at its terminals multiplied into 

 the current passing through it, and is generally expressed 

 in volt-amperes, i.e., the product of the potential (or 

 electric pressure) expressed in the practical units or volts; 

 multiplied into the strength of the current in amperes. 

 Thus the "efficiency'' of a glow lamp depends on the 

 pressure at which it is worked and the strength of the 

 current which circulates through it. The length of 

 time which such a lamp lasts (or its "life") and 

 its efficiency are intimately connected. The overlooking 

 of this fact has led in the past to numerous misleading 

 or partial statements, even in technical " journals. 

 To know the quality and goodness of a lamp we 

 must be acquainted, not only with the number of 

 hours it has existed w'hile giving out light, but also with 

 the amount of energy which has been employed in pro- 

 ducing this light. As an example, I may take an instance 

 given by Sir David Salomons. He says he has hundreds 

 of lamps which have run over 3000 hours, and he attributes 

 the long life of his lamps to his method of obtaining con- 

 stant electro-motive force round his circuit. The pressure 

 (or electro-motive force) in this instance is kept between 

 the limits of 98 volts and 100 volts, and the results have 

 exceeded expectations. Here on the average it has been 

 found that 10-candle power Edison-Swan lamps, meant to 

 be worked at a pressure of 100 volts, give a light whose 

 intensity is equal to 15 candles at 08 volts and 17 candles 

 at 100 volts. 



For practical purposes, another important factor comes 

 in, and one which determines which lamp shall be used in 

 a given case, and this is the nM of the production of the 

 energy required. This last factor varies with circum- 

 stances, the sources of power being so dift'erent — e.<i., 

 waterfalls used to drive dynamo machines, secondary 

 batteries previously charged up, primary batteries, Ac. 



The eoinmeveial ejfieieney of a lamp depends not only on 

 the amount of energy required to work it, but also on the 

 cost of this energy. 



If we take a fixed quantity of gas and compare the 

 result — -first, when it is used to drive a gas engine which 

 works a dynamo machine, and thus produces a current 

 through a set of incandescence lamps, or, .leroii'lli/, when 

 the gas is burned at once, and thus used directly as an 

 illuminant, it is found that the light-giving efficiency of 

 the electrical method is three times that of combustion. 

 But the cost of the electrical method is at present greater 

 on account of the first expense, and then the maintenance 

 of the gas engine and dynamo. 



The advantages of using electricity as a means of light- 

 ing in mines are very noticeable. The only suitable lamps 

 for such a purpose are those which are portable. Hence, 

 if electricity is to be used at all, the lamps employed must 

 be incandescence lamps. When these are made use of, the 

 work becomes more sure, more regular, and more rapid, 

 and it is even claimed that the moral nature of the miners 

 is improved when they work surrounded by the brighter 

 and more ngreeable light of electric glow lamps. The 



