December 1, 1891.] 



KNOWLEDGE 



225 



Flinders has, however, happily been adopted, and is a more 

 rational name than those of Europe, Asia and Africa, which 

 originally denoted only three small plains ; the plain round 

 Thebes, the plain round Ephesus, and the plain round 

 Carthage. 



('I'll lie nmtinui'iL] 



SOME PRACTICAL APPLICATIONS OF 

 ELECTRICITY. 



By .J. .T. Stewaet ifniiiu'ilij I iiiinui.stnitiir nj I'hi/.sirs 



lit. ['nh-i-rsilif Ciilli'i/r, London). 



[CanthiHi'il f'roiii piuje 175.) 



III. — Arc Lamps. 



ABOUT the year 1802, Sir Humphry Davy, by 

 sending an electric current from a powerful 

 voltaic battery of 2000 cells through two sticks 

 of wood charcoal placed a short distance apart, 

 obtained a brilliant discharge between the two 

 charcoal points. The rush of Electricity produced a bright 

 stream of light, shaped like a bow, in the interval between 

 the ends of the charcoal, and hence the name of " Voltaic 

 Arc " was given to it by Davy. Some years later he 

 exhibited the eifect before an audience at the Eoyal 

 Institution. Of course, the soft charcoal was rapidly 

 disintegrated, and from this cause, and also on account of 

 the great expense at which it was produced, when the 

 battery of cells was the only available means of generating 

 the current, this first example of the electric light could 

 not be applied to any practical or commercial use. 



It was only when the introduction of dynamo machines 

 rendered possible the production of powerful currents at 

 comparatively small cost that there was any prospect of 

 Electricity being used for purposes of illumination. Those 

 to whom we owe the first production of the electric arc do 

 not seem to have had much expectation that it would 

 prove a general means of obtaining artificial light, and in 

 a description of the voltaic arc, published so late as the 

 year 1880, it is described as painfully, and even dangerously 

 intense, and is said to dazzle rather than illuminate. 



When a strong current of Electricity forces its way 

 across a gap in a metallic wire which it is traversing, 

 the air in the space between the ends of the conductor, 

 opposing as it does a great resistance to the passage of 

 the current, is raised to a very high temperature, minute 

 particles of the metal of the wire are driven off, and these, 

 wuth any dust which may be ni the air, are raised to a 

 white heat, and thus a luminous bridge is formed between 

 the two broken ends of the wire. The heat generated in a 

 conductor in a given time by the passage of a current of 

 Electricity is proportional to the square of the current and 

 to the resistance of the conductor. When the current 

 remains the same the heat produced varies simply with 

 the resistance. In the above case the resistance of the 

 wire conveying the current is small, while that of the air 

 gap is very groat ; thus there is little heat developed in the 

 wire itself, but at the break in the circuit where the current 

 passes through air the heat developed is very great, being 

 suflBciont to make white-hot the air and metallic vapour in 

 its patli. No metallic conductors would be durable enough 

 to allow of a permanent arc being set up between them ; 

 it is only with an infusible substance such as carbon that 

 this can be done. The source of the light in the electric 

 arc is in the incandescent particles of carbon driven oft' from 

 the carbon poles. The first step in the progress to the useful 



application of the arc was the employment of hard carbon, 



taken from the deposit whicli forms inside the gas retorts 

 during the manufacture of coal gas. Pencils of this 

 material employed instead of Davy's soft charcoal gave 

 a much more permanent arc, but the light was still 

 uncertain and liable to much flickering, owing to inequali- 

 ties in the substance of the carbon. 



The voltaic arc is not an example of disruptive 

 discharge, as in the case of the spark from an ordinary 

 electric machine or Leyden jar, but seems rather to be an 

 instance of conduction, the air at the high temperature to 

 which it is raised becoming a conductor of Electricity. 

 Thus, also, when once the arc has been started between 

 the carbon points, they may be removed to a greater 

 distance apart without the disappearance of the arc. The 

 passage of incandescent particles of carbon from one 

 terminal to the other can be shown by throwing a magni- 

 fied image of the heated points on to a screen by means of 

 a lens. On observing the image thus produced particles are 

 seen to be traversing the arc, sometimes in one direction 

 and sometimes in the other, but mostly from the positive 

 to the negative carbon. The positive carbon wears away 

 twice as rapidly as the negative one. Its end becomes 

 hollowed out into a crater-like cavity, while the negative 

 carbon preserves its pointed appearance. When the carbon 

 points are placed in a vacuum, this difi'erence in behaviour 

 is more marked. The image of the points projected on the 

 screen will also show the appearance of round globules at 

 the sides of the glo\\'ing carbons ; these are due to melted 

 portions of silica and other impurities in the carbon. 



The voltaic arc excels, both in temperature and bright- 

 ness, all other artificial sources of heat, and, by its means, 

 some of the most refractory substances, which had resisted 

 all other attempts to melt them, have been fused and some 

 volatilized. 



However, a large proportion of the rays emitted do not 

 affect the eye as light, and thus a considerable part of the 

 energy in the arc is wasted for the purpose to which it is 

 to be applied, that of illumination. In fact, we as yet know 

 of no artificial method of economically producing energy in 

 the form of waves of light ; a notable portion of the energy 

 we get seems necessarily expended in generating the dark 

 heat rays, and those ultra-violet rays which do not produce 

 the effect of light to our eyes. The only really economical 

 light known where nearly all the energy is obtained in 

 luminous waves, is that of the humble spark of the glow- 

 worm ; in this the heat is negligibly small and the 

 luminous part marvellously great. 



The arc, like other conductors conveymg currents, is 

 acted on by magnets, and by using a powerful magnet the 

 arc may be driven out to one side and made to take a 

 pointed form like a blow-pipe flame. 



One of the principal factors iu the production of a good 

 and steady light from the voltaic arc is the use of carbons 

 of a compact and uniform structure. The production of 

 such carbons is the object with many manufacturers, most 

 of whom observe considerable secrecy as to the actual 

 methods they employ. In one form of carbon the stick is 

 pierced along its length to form a tube, which is afterwards 

 closed at one end and filled up by a solution of certain 

 materials containing in suspension a fine powder of the 

 same sort as that which composes the original rod. This 

 is driven in under high pressure, and has as its result the 

 displacing of the gases which are contained in the carbon, 

 and which it is important to remove. The hollowed carbon 

 rod is in this way by degrees tilled «p, and at length a very 

 compact sort of carbon is obtained. 



As an example of the methods used to obtain a suitable 

 carbon, the process employed iu the manufacture of Carre's 



