124 



KNOWLEDGE 



[July 1, 1892. 



subject, and those whose interest has thus been aroused 

 may perhaps have the good fortune to discover new and 

 unsuspected instances of one or other of these types of 

 protective resemblances. 



T 



SOME PRACTICAL APPLICATIONS OF 

 ELECTRICITY. 



By J. J. Stewart. 



(Continued from page 2iQ, Vol. XIV.) 



IV. — Incandescence Lamps. 



(HERE are two important types of lamp used in 

 illumination by means of electricity — that of arc 

 lamps, described shoitly in my last paper, and 

 that in which the immediate source of light takes 

 the form of a thread of carbon raised to a white 

 heat. In describing this second form of lamp it may lead 

 to clearness to consider first, as an example, that special 

 kind of incandescence lamp knoT\Ti as Swan's ; for the 

 difference between the various adaptations of glowing 

 carbon to lighting purposes is one of detail merely, the 

 main principle being the same in all. 



Mr. Swan tells us that for long it had seemed to him 

 that if ever the electric light was to become generally 

 useful it would be through some application of the incan- 

 descence of carbon ; and he made a long series of careful 

 experiments in his endeavour to hit upon some method 

 wherebv incandescent carbon might be rendered an illu- 

 minating agent which would last and give a permanent 

 light. 



It is now thirty years since Mr. Swan performed the 

 following experiment in his investigation of this subject. 

 He caused a number of strips of card and paper to be 

 surrounded by charcoal and then put in a crucible, which 

 was afterwards raised to a white heat in a pottery kiln. 

 These pieces of cardboard were in the form of an arch 

 half an inch in length and a quarter of an inch across. They 

 were changed to strips of carbon by the intense heat of 

 the furnace, and their ends were fixed in square carbon 

 blocks by means of small clamps. When these carbon 

 threads, thus prepared, were placed in glass vessels and the 

 air exhausted by means of an air pump, Mr. Swan was 

 delighted to see the threads brighten up witli a ruddy 

 glow whenever an electric current from a voltaic battery 

 of 40 or .50 cells was sent through them, and he felt 

 confident that all that was needed was a stronger current m 

 order that they should give out a brilliant white light. 

 Swan believed that this was the first occasion on which 

 carbonized paper was made use of in the construction of 

 an incandescence electric lamp. At that time (somewhere 

 about the year 1800) the cheajiest source of the electric 

 current was the voltaic battery, but, as in the case of are 

 lamps, the introduction of the dynamo machine with its 

 powerful currents gave a great impetus to the attempt to 

 utilize glownig carbon. 



After Swan's earliest experiments the matter rested, so tar 

 as he was concerned, for seventeen years, till about the year 

 1877, when he was again led to take up the subject. Not 

 only was there a difficulty at first iu getting a strong 

 current at small cost, but the limits attainable in producing 

 a high vacuum were nothing like what they became, after 

 the introduction of the Sprengel air pump. The durability 

 of the incandescent filament of carbon depends very much 

 on the perfection of the vacuum obtained in its. containing 

 globe ; because in the presence of the oxygen of the 

 residual air the carbon consumes away more or less 



rapidly. Crookes had shown, in his beautiful experiments 

 with the radiometer, how good a vacuum could be attained 

 by means of the Sprengel pump, and Mr. Swan adopted 

 this method of producing a high vacuum in his glass 

 globes. In conjunction with Mr. Stearn, who had had a 

 large experience in the use of the Sprengel pump, he 

 mounted some of his carbonized paper filaments in the 

 globes provided for them, and then exhausted the globes 

 of ail- as completely as possible. As showing the impor- 

 tance of attention to minute details, it is interesting to 

 note that in this case it was found that the carbon ab- 

 sorbed some of the air around it while it was cold, and 

 then gave out this air when it was heated strongly by the 

 passage of the current, thus partially destroying the care- 

 fully prepared vacuum. The device of heating the carbon 

 threads strongly by passing through them a current irhlle 

 the iiroceas of exhaustion was ijolnij on was made use of, and 

 thus the air entangled in the carbon was driven out before 

 the globes were finally sealed. It is by careful attention 

 to such minute particulars as these, which less earnest 

 investigators might have overlooked, that the present 

 brilliant success in electric lighting has been reached. 

 The wires leading the current to the lamp are in connec- 

 tion with supporting clips, and great care has to be used 

 in order to make a good contact between the threads of 

 carbon and the metallic clip, and such an one as shall be 

 able to resist the high temperature to which the junction 

 is exposed. In Swan's lamps the carbons were thickened 

 at their ends, and, in some of 

 the first trials, electrotyping 

 and hard soldering of the ends 

 of the carbon to platinum 

 was applied. 



In 1878 Edison took up 

 the subject of incandescence 

 electric lighting. After unsuc- 

 cessfully attempting to make 

 a durable conductor out of a 

 mixture composed of infusible 

 earth with carbon and me- 

 tallic substances, he proceeded 

 to use platinum, still with- 

 out success. Afterwards he 

 went on, with his character- 

 istic unflagging energy, to try 

 "carbon wires," ashediscover- 

 ed that a thread of ordinary 

 sewing cotton, when properly 

 carbonized, remained un- 

 broken for a long time, 

 even at extremely high tem- 

 peratures, when it was placed 

 in a bulb exhausted of air 

 to such an extent that the 

 I pressure was reduced to one- 

 ' millionth of an atmosphere. 

 i In some of his trial lamps 

 he emploj'ed filaments made 

 i from Bristol board, but 

 ! ultimately he chose filaments of bamboo as. most suitable 

 ' for the " Edison lamps." 



There were experimenters in this field of incandescence 

 hghting before either Swan or Edison took up the subject. 

 M. Jobard, in 18'38, suggested the use of carbon; 

 placed in a vacuum, as a conductor, and this idea 

 was carried out twenty _years later, iu 1858. Early workers 

 were King, iu our own couutry, in 1815, and Starr in 

 ' America. Mr. Mattieu ^Yilliams was working with King 

 I and Starr, and he says : " We had no difficulty in obtain- 



FiG. 1. — Shows the Swan 

 Lamp connected up and ready 

 for use. The spiral spring s 

 (shown apart from the lamp in 

 Fig. 2) is compressed round the 

 neck of the globe, and holds it 

 with a firm yet flexible grip, the 

 electrical contact being perfect, 

 andthe whole connection leaving 

 little to be desired in point of 

 neatness and efficiencT. 



