GALVANISM. 



and produce an explosion in a moment This 

 principle has been employed with great success in 

 the construction of cartridges for use in blasting 

 operations, and they have become very common 

 on account of their safety and certainty of working. 

 Luminous Effects. When the wires from a 

 powerful battery are brought near to each other, 

 no current passes till they touch. On separating 

 them, a bright spark passes, due, as we shall after- 

 wards see, to induction. Nor does the current 

 cease after a small separation. It forces its way 

 across the short interval of air, producing a 

 brilliant light, and a heat so intense as to melt the 

 most infusible metals. The ends of the wires will 

 therefore be burned away, and we must seek for 

 some more infusible conductor, in order that the 

 current and the light may continue. This is found 

 in the various kinds of carbon, such as charcoal, 

 coke, &c. By far the best form of carbon for this 

 purpose is the charcoal deposited in the retorts of 

 gas-works. The carbon is formed into pointed 



Fig. 27. 



sticks or pencils, to fit two metal-holders such as 

 are shewn in fig. 27. They can be adjusted to 

 any distance by sliding-rods, to which they are 

 fixed, and which communicate with the wires of 

 the battery. When the current is on, and the 

 points separated by one-tenth of an inch or so, a 

 most dazzling light is produced, rivalling that of 

 the sun in purity and splendour. It is too bright 

 for the eye to examine it ; but an image of the 

 points can be cast on a screen, and then there will 

 be seen, as in the figure, an arch of flame between 

 the points. This arch of flame, or the voltaic arc 

 of the electric light, is composed of white-hot 

 particles of charcoal flying from the one pole to 

 the other. The positive pole gradually gets hollow, 

 v> hile the negative remains pointed, apparently by 

 the addition of matter. 



The heat of the voltaic arch is the most intense 

 that can be artificially produced. Platinum melts 

 in it like wax in the flame of a candle. Quartz, 

 lime, and magnesia are fused by it ; and the 

 diamond itself is not only fused, but reduced to a 

 black mass like coke. 



As the carbon points waste away, and the posi- 

 tive much more rapidly than the negative, some 

 arrangement is necessary to keep the points at 

 such a distance that the current can pass, and 

 so the light continue. This is the object of the 

 ilectric lamp. For an account of the principal 

 lamps in which the voltaic arc is used, and the 

 other species called incatuiescent lamps, see page 

 496 of the present volume. 



Great progress has recently been made in 

 methods for applying the electric light to the 

 illumination of streets in cities ; and its close 

 rivalry has led to more economic and effective 

 methods of using coal-gas, which for ease in use 

 and cost has still for general purposes the superi- 

 ority. The electric light has, however, been used for 

 streets and public buildings. It has also long been 



used with excellent effect where a limited space 

 had to be lit up for a few nights, as in the con- 

 struction of bridges across rivers, and the like. 



For lighthouses it is specially suited, and has 

 been successfully adopted in many of them. That 

 at Dungeness has been lit up with it since 1862 ; 

 and that at La Heve, near Havre, since 1863. It 

 is decidedly superior to oil-lamps in such cases, as 

 its illuminating power is very great, and remark- 

 ably penetrating in hazy weather. 



Chemical Effects. When a current passes 

 through a liquid, composed of two or more simple 

 elements, there is in general a separation of the 

 elements or a decomposition of the liquid. Elec- 

 trolysis is the name given to this decomposition, 

 the liquid so broken up being called the electrolyte. 



The analysis of water by the current may be 

 taken as the type of electrolytic action, and a very 

 simple method of shewing this is seen in fig. 28. 



Two slips of platinum 

 are soldered to two copper 

 wires, which are passed 

 through a cork at the 

 bottom of a glass basin. 

 The basin is filled with 

 water, slightly acid to 

 make it conducting. 

 Now when the platinum 

 plates are connected with 

 the wires from a battery, 

 the liquid is polarised, 

 oxygen going to the 

 positive plate, and hydro- 

 gen to the negative. If 

 the current be strong 

 enough, these elements 

 will be separated from 

 the liquid, and as plat- | 

 inum combines with | 

 neither, each will bubble 

 up at its platinum pole. 

 They may easily be col- 

 lected separately by filling two glass tubes with 

 the acid water, and inverting them over the plates. 

 Doing this, we find that the hydrogen is produced 

 in twice the quantity of the oxygen ; and we learn 

 from this the important fact, that elements are 

 decomposed by electrolysis in the very same pro- 

 portion as they are united in the compound, for 

 water, we know, has two atoms of hydrogen for 

 each of oxygen. 



Faraday called the two poles at which the 

 elements appear, the electrodes, that is, electric 

 ways, the positive pole being called by him also 

 the anode (or up way), and the negative, the cathode 

 (or down way). The element that goes to the 

 anode he called the anion (or up-going one), and 

 that to the cathode, the cation (or down-going 

 one). In the case of water, oxygen is the anion 

 or electro-negative element, and hydrogen the 

 cation or electro-positive one. 



It is to be noted that a single cell can never 

 decompose water, or any electrolyte ; at least two 

 are required to overcome the polarisation of the 

 plates or counter-current which the anion and 

 cation tend to set up. The greater the strength 

 of the current, the more rapidly are the elements 

 liberated ; and the quantity of gas given off corre- 

 sponds exactly to the current power. 



Faraday turned this to practical use as a vol- 

 tameter, or current measurer, and its value is that 



Fig. 28. 



