Part 11 (1/2)

INCANDESCENT LAMP

If you take a piece of very fine iron wire and lay it across the terminals of an accuenerated by its resistance to the current A piece of fine platinuive out an intense light Here we have the principle of the glow or incandescent lamp--namely, the interposition in an electric circuit of a conductor which at once offers a high resistance to the current, but is not destroyed by the resulting heat

In Fig 80 is shown a fan propelling liquid constantly through a pipe

Let us assureat friction on the inside of the pipe At the contraction, where the speed of travel is reater than elsewhere in the circuit, most heat will be produced

[Illustration: FIG 80--Diagrah a pipe]

In quite the early days of the glow-la, and fila away of the carbon by colass bulb from which practically all air has been sucked by a

[Illustration: FIG 81--The electrical counterpart of Fig 80 The filament takes the place of the contraction in the pipe]

The low-lamps is now an important industry One brand of lamp[20] is made as follows:--First, cotton-wool is dissolved in chloride of zinc, and forh a fine nozzle into a settling solution which hardens it andwashed and dried, it is wound on a pluo rod and baked in a furnace until only the carbon eleh It is next removed from the rod and tipped with two short pieces of fine platinum wire To ed in benzine and heated to whiteness by the passage of a strong current, which deposits the carbon of the benzine on the joints The filalass receiver of an air-pump, the air is exhausted, hydro-carbon vapour is introduced, and the filah it to make it white hot Carbon from the vapour is deposited all over the filament until the required electrical resistance is attained The filament is now ready for enclosure in the bulb When the bulb has been exhausted and sealed, the la departlass) are soldered to a couple of brass plates, which make contact with two terminals in a lamp socket Finally, brass caps are affixed with a special water-tight and hard ce, instead of a contraction at a point in the circuit, there is an actual break of very s from a dynamo's terminals we attach two carbon rods, and touch the end of the rods together The tips becohtly, atoative rod in a continuous and intensely luminous stream, which is called an _arc_ because the path of the particles is curved No arc would be formed unless the carbons were first touched to start incandescence If they are separated too far for the strength of the current to bridge the gap the light will flicker or go out The arc lamp is therefore provided with a mechanism which, when the current is cut off, causes the carbons to fall together, gradually separates them when it is turned on, and keeps theh which a current passes to draw an iron rod into its centre Soh a coil, into which projects one end of an iron bar connected with one carbon point A spring norether when no current flows As soon as current circulates through the coil the bar is draards against the spring

SERIES AND PARALLEL ARRANGEMENT OF LAMPS

When current passes fro 82, the lamps are said to be in _series_ Should one lao out But where arc laed a special mechanism on each lamp ”short-circuits” it in case of failure, so that current may pass uninterruptedly to the next

[Illustration: FIG 82--Incandescent la 83 shows a number of lamps set _in parallel_ One terminal of each is attached to the positive conductor, the other to the negative conductor Each lae, and does not affect the efficiency of the rest _Parallel series_ signifies a combination of the two syste 83, two or roups froe

[Illustration: FIG 83--Incandescent lamps connected in ”parallel”]

CURRENT FOR ELECTRIC LAMPS

ThisThe former is commonly used for arc lamps, the latter for incandescent, as it is easily stepped-down froh-pressure mains for use in a house Glow-lamps usually take current of 110 or 250 volts pressure

In arc lamps fed with direct current the tip of the positive carbon has a bowl-shaped depression worn in it, while the negative tip is pointed

Most of the illumination comes from the inner surface of the bowl, and the positive carbon is therefore placed upper current, of course, affects both carbons in the same manner, and there is no bowl

The carbons need frequent renewal A powerful lamp uses about 70 feet of rod in 1,000 hours if the arc is exposed to the air Some lalobes perforated by a single sh preventing a vacuu is the art of coating metals with metals by means of electricity Silver, copper, and nickel are the enerally deposited The article to be coated is suspended in a che 84 shows a very si, say, an acidulated solution of sulphate of copper A spoon, S, hanging in this froative element, Z, of the battery, and a plate of copper, P, with the positive element, C Current flows in the direction shown by the arrows, from Z to C, C to P, P to S, S to Z The copper deposited froradual dissolution of the plate, so that the latter serves a double purpose

[Illustration: FIG 84--An electroplating outfit]

In silver plating, P is of silver, and the solution one of cyanide of potassium and silver salts Where nickel or silver has to be deposited on iron, the article is often given a preliood junction with either of the first two metals, but has an affinity for copper

[17] Froe”