THE POPULAR EDUCATOR. 



Silver . 

 Copper 

 Gold . 

 Sodium 

 Ziuc . 



the same operation gone through with it as with the first. Thus, 

 if in the one case we find that 3 feet 6 inches of the wire is 

 transferred to A, and in the other case 5 feet 6 inches, we shall 

 at once see that the resistances offered by the two wires bear to 

 one another the ratio of 3j to 5.j, or 7 to 11. 



In this way the resistances offered by various metals to the 

 passage of an electric current have been carefully ascertained, 

 and it is found that at ordinary temperatures silver is the 

 best conductor. The conducting powers of different metals 

 vary, however, with their temperature, so that it is almost 

 impossible to give a table which shall show their relative 

 powers with any degree of accuracy. The purity or otherwise 

 of the metals likewise has a very great influence on their con- 

 ducting powers, and hence the tables given by different 

 observers vary very greatly. The following determinations are 

 those made by Professor Matthieson : 



lOO'O Potassium. ^0'8 German Silver 77 

 77'4 Iron . . . 14'4 Antimony . . 4'3 

 55-2 Tin . . . 11 '4 Mercury . . 1'6 

 37'4 Platinum . 10 '5 Graphite . . 0'069 

 27-4 Lead . . 77 Gas Coke . . 0'038 



Other observers place copper at the head of the list. 



When we come to compare the conducting power of liquids 

 with that of the metals, we find that it is small indeed. Thus, 

 if we set down the conducting power of silver at 100,000,000, 

 that of a saturated solution of common salt will only be 31, 

 that of sulphuric acid having a density of 1*24 will be 132'75, 

 and that of distilled water 0'013 only. 



Having now seen the mode in which we obtain the electricity, 

 and the way in which we can measure its intensity, we must 

 notice the principal effects which can be produced by means of 

 it. These are so numerous, that the best way of examining 

 them is to divide them into classes ; we shall therefore consider, 

 first of all, the luminous effects of the electric current. 



If a number of cells of any powerful battery are set up and 

 connected together, no effect will be observed until the wires 

 from the terminals are brought into contact, unless, indeed, a 

 local circuit is caused by the batteries not being sufficiently 

 insulated. As soon, however, as the wires are brought to- 

 gether, and then separated to a slight distance, a very brilliant 

 green light will be seen between them, and the wires will very 

 speedily become intensely heated. The light, too, is so dazzling, 

 that it is almost impossible to hold the wires steady. If they 

 are twisted round small pieces of gas carbon, and the current 

 allowed to pass between these, a much more brilliant white -light 

 will be seen, and this is the well-known electric light. 



A simple arrangement for showing this is represented in 

 Pig. 30. Two brass uprights, A and B, carry at their upper ends 

 crayon holders, in which the pieces of carbon may be placed 

 and held firmly by means of the loose rings on them. One of 

 t-iese, B, instead of being fixed to the board, is screwed to a 

 metal plate, c, one edge of which is cut into a rack so that it 

 may be moved backwards or forwards by means of the milled 

 head and pinion. In each support a hole is drilled, into which 

 the battery wire may be inserted, and held by means of small 

 screws. 



Now connect the battery wires with this, and by means of 

 the milled head bring the charcoal points so as to meet ; they 

 will at once become red-hot, and on slowly separating them, we 

 shall obtain the light in all its brilliancy. The length of the 

 arc of light between the points will depend on the power of the 

 batteries ; it will not, however, be by any means large. If forty 

 or fifty cells of Grove's battery are used, the distance should be 

 a little over half an inch, but it will become less after the 

 batteries have been worked for a short time. As soon as the 

 distance becomes too great for the electricity to pass, the light 

 will be extinguished, but the points will continue to glow for a 

 short time, owing to the intense heat to which they have been 

 exposed. The light will not, however, be renewed till the points 

 actually touch again. With an apparatus of this kind the light 

 is not by any means constant, for particles are continually 

 given off from the positive pole, which, therefore, wastes away 

 till the distance becomes too great for the current to pass, and 

 the light is then extinguished. We must not, however, suppose 

 that the points burn away and thus produce the light ; it arises 

 entirely from the passage of the current between the poles and 

 the intense heat produced thereby. In proof of this we may 

 place the charcoal points inside a,n exhausted receiver, and shall 



find that the light is not at all impaired in brilliancy. We 

 may even place it under water, and shall find then that it still 

 burns almost as brightly as before, proving conclusively that 

 the light is not produced by the carbon combining with the 

 oxygen of the air. 



If we carefully examine the light using a dark glass to pro- 

 tect the eyes we shall observe that the electric fluid does not 

 pass in a straight line between the points, but forms a luminous 

 arc, which at times seems to rotate around the points. It will 

 be further seen that this arc is attracted if a magnet be brought 

 near to it. Another strange fact in connection with this light 

 is, that it may very easily be extinguished by blowing on it. 



If, however, we want to understand fully the changes going 

 on between the points, we must repeat the beautiful experiment 

 known as Foucault's, which consists in throwing a magnified 

 image of the points on a screen. This is done by placing the 

 light in an ordinary lantern, so that the points are just in the 

 focus of the lens ; we shall then have a brilliant image of them 

 projected on the screen, and presenting the appearance shown 

 in Fig. 31. The negative pole will be somewhat pointed, and 

 will be observed to increase slightly by the addition of particles 

 from the positive pole. This, on the other hand, becomes 

 hollow or cup-shaped, and wastes more rapidly. The surface of 

 both poles will frequently appear covered with small bubbles, 

 which break from time to time. These arise from some of the 

 silica, which is usually present in the carbon, fusing and boiling 

 on its surface. The arc of light with its varying colours will 

 also be seen playing round the points. 



If we remove the carbon poles, and in their place employ 

 poles composed of different metals, we shall obtain variously 

 coloured flames. The heat is so great that the metals are 

 actually fused and even turned into vapour. When two three- 

 penny pieces, or, better still, two points of silver, are used, 

 dense green fumes will be given off, and the hue of the light 

 will be very remarkable. Points of zinc and of various other 

 metals may likewise be employed, and in each case the appear- 

 ance of the light will be different. The heat, too, is so intense 

 that globules of the metal will often drop down, being melted 

 by the current. 



Even when the hardest graphite is used for the points, they 

 consume so rapidly that, with the apparatus already described, 

 the light cannot be kept steady for more than a few minutes, 

 and hence for all practical purposes as for the illumination of 

 towns or for the lighthouse lanterns it is almost useless. Many 

 different plans have accordingly been suggested for rendering 

 the light steady. It is, however, a somewhat difficult matter to 

 accomplish this, as it is necessary first of all to cause the poles 

 to move towards one another with a speed closely corresponding 

 to that at which the points are consumed, and this varies with 

 the quality of the points and the strength of the battery. As 

 a rule, however, the positive pole must move at double the 

 speed of the negative one. Another thing requisite is to cause 

 the points to come together immediately the light is in any way 

 interrupted, and then to separate again to such a distance as to 

 cause the greatest brilliancy c" light, avoiding, on the one hand, 

 so small a separation as to give only a short arc of light, and, 

 on the other hand, so great a separation as to extinguish it by 

 interposing too great a distance for the current to pass over. 



One of the simplest, though by no means the best, modes of 

 accomplishing this is by the arrangement shown in Fig. 32. 

 This apparatus will help us to understand the action of others 

 which will shortly be referred to. It consists of a brass sup- 

 port, which carries the upper pole, and is usually so constructed 

 that the position of the carbon may be adjusted by means of 

 sliding tubes. The wire from the negative pole of the battery 

 is brought to this support. On the same stand is a large 

 bobbin, wound round with insulated wire, one end of which is 

 brought to the binding screw seen on the right, while the other 

 is so arranged that it may be in contact with the carrier of the 

 lower pole which moves inside the bobbin. 



A small pulley is fixed at the lower end of this carrier, and a 

 cord is passed from one side of the coil, under this pulley, then 

 up again and over a second pulley on the fixed upright, a 

 balance weight being fastened to it, so heavy as just to keep 

 the pole in contact with the upper one. 



The instrument is now put in the circuit, and the current 

 passes up the brass support to the upper pole, along that to the 

 lower one, then round the coil or bobbin, and so to the binding 



