CONDUCTORS AND NON-CONDUCTORS.

thickness be diminished, or the quantity of electricity passing over it be augmented, or, in general, if the ratio of the electricity to the magnitude of the space afforded to it be increased, the conductor will be found to undergo an elevation of temperature, which will be greater the greater the quantity of the electricity and the less the space supplied for its pass

age.

These heat effects are manifested in different degrees in different metals, according to their varying conducting powers. The worst conductors of electricity, such as platinum and iron, suffer much greater changes of temperature by the same charge than the best conductors, such as gold and copper. The charge of electricity, which only elevates the temperature of one conductor, will sometimes render another incandescent, and will vaporize a third.

The better the conducting power of a substance, the lower its electrical resistance, on the other hand the lower its conducting power the more its electrical resistance. A very small percentage of carbon, zinc, iron or other impurity greatly reduces the conducting power of copper, even to one-half.

Iron is frequently used for conductors. This is owing to its comparative cheapness.

In case of wires, or other conductors, passing through walls, etc., they must be covered with a layer of coating of cotton or silk, wrapped or braided over the wire.

When wires or cables are to be used under water, they must be made impervious, and great care must be used to prevent

CONDUCTORS AND NON-CONDUCTORS.

the water from penetrating and thus injuring the insolationthe insulating material must possess a good degree of nonconducting power.

It would be a wise plan, and should always be required, that where there is any considerable investment in copper to have submitted by the manufacturer a sample piece of fixed dimensions which can be tested for its conductivity and where it is used for overhead conductors also tested for tensile strength. There are a number of processes used in the production of copper which do not eliminate from the copper those impurities which seriously affect its resistivity. They are not apparent from anything but a test.

ELECTRICAL MEASUREMENTS.

and

Electricity has been termed the science of measurements,

The electrical industry, more than any other, necessitates accurate and varied measurements. Fortunately, the average engineer and electrician may quite readily acquire the knowledge and skill to make all ordinary tests that are necessary, and the cost of needful apparatus is not excessive.

Measurement has been defined as the determination of the value of quantities.

Every system of measurement is based upon some experimental fact or law. An electric current can (1) cause a deposition of metals from their chemical solutions; (2) heat the wire that it flows through; (3) attract (or repel) a parallel neighboring current; (4) accumulate as an electric charge that

NOTE-Scales are now made of such a nice adjustment that they will weigh anything, to the smallest hair plucked from the eyebrow. They are triumphs of mechanism, and are enclosed in glass cases. Two pieces of paper of equal weight put in the scales, and an autograph written in pencil on either piece will cause the other side to ascend, and the needle which indicates the division of weight even to the ten millionth part of a pound and less will move from its perpendicular. A signature containing nine letters has been weighed and proved to be the fifteen thousand five hundredth part of an ounce, troy.--Practical Engineer, London, Fng.

ELECTRICAL MEASUREMENTS.

can repel (or attract) a neighboring charge of eletricity; (5) produce in its neighborhood a magnetic field, that is to say, can exert a force upon the pole of a magnet placed near it, as, for example, in galvanometers; the fifth of them is made the basis in the system now adopted by international agreement; and it is the best because, firstly, it connects the electrical units with the magnetic ones, and, secondly, it is closely connected with the mechanical units, enabling the mechanical values of the electrical quantities to be readily computed.

Instruments for measuring currents of electricity are of many styles. As a mysterious and invisible element is dealt with, the measurement is indirect. The effects of currents of various pressures and volumes are what are measured, not the currents themselves.

NOTE.-One of the most sensitive instruments to the effect of the current is the ordinary compass needle. Now if a wire through which an electric current is passing be laid directly above the needle and parallel to it the needle is turned aside from its north and south position. If the current flows in one direction the needle will turn to the right. If the current is from the other direction it will turn to the left.

When the current in the wire is very feeble the needle will be deflected very little from its natural position. If the current be very heavy the needle will be turned aside much more. In order to effect the needle with very small currents the influence must be increased and this is done by winding a hollow, flat coil about a space large enough to contain the needle. This gives many turns of wire above and below the needle and the effect upon the needle is multiplied in proportion to the number of turns.

Upon this principle depends the working of the galvanometer, the voltmeter and ammeter. All are constructed upon the same principle but the voltmeter usually has many turns of very fine wire while the amImeter has but a few turns of very heavy wire.