Electric circuit

“ As it is a law of every electric circuit that the rate of transformation of electric energy to heat or work in each of its several parts is directly proportional to the drop of voltage therein, it follows that a drop of 500 or 1,000 or 2,000 volts in the conductors of a 10,000-volt transmission line at full load would correspond to a power loss of five to ten or twenty per cent respectively. ”

“ Whenever, therefore, we find these two states of heat and magnetism present together in and round a wire, we may take it as an indication that it forms part of a circuit through which an electric current is flowing. An electric current is a physical state or condition which can only exist in or all along a closed path which is called an electric circuit. ”

“ Fig. 3.—Three-junction Thermo-electric Circuit.
A point of practical utility in thermo-electric work is the fact that if a wire be interrupted by a length of other metal, as indicated at C in fig. 3, no current will be set up in a circuit if both joints are equally heated, as the electromotive forces generated at each junction [26] are in opposition. It is thus possible to interrupt a circuit by a plug-key or switch, without introducing an error; always provided that an even temperature prevails over the region containing the joints. ”

“ The connections between battery push button and bell form a complete circuit. In Fig. 40 B represents a battery, usually of dry cells, B' represents the bell, and P represents the push button. The electric circuit is "open," (that is, there is a break in the conductor) at P until some one "pushes the button," that is, simply pushes against a spring so as to cause a piece of metal to bridge the gap in the conductor. Then we say the circuit is "closed."
[74]
Fig. ”

“ Indeed, a close examination will show that wherever power is expended in developing current in a circuit, cutting lines of force, the energy expended is first employed in stretching the lines, which thus receive the energy required to permit them, in shortening, to cut the conductor and set up currents in the electric circuit in accordance with the potential difference developed in that circuit and its resistance. ”

“ Thus the Leyden jar L and associated coil P shown in Fig. 82, p. 271, constitutes an electric circuit, having a certain capacity measured in units, called a microfarad, and a certain inductance, or electric inertia measured in centimetres. The capacity of the circuit is the quality of it in virtue of which an [291] electric strain or displacement can be made by an electromotive force acting on it. The inductance is the inertia quality of the circuit, in virtue of which an electric current created in it tends to persist. ”

“ How quickly a spoon inserted into a hot cup of tea becomes uncomfortably hot, though the hand can be held very close to the liquid without feeling more than a gentle warmth. And we all have noticed that the very least air-gap in an electric circuit effectively breaks a current capable of traversing miles of wire. If the current is so intense that it insists on passing the gap, it leaps across with a report, making a spark that is at once intensely bright and hot. Metal wires are to electricity what speaking tubes are to sound; they are as it were electrical tubes through the air and ether. ”

“ The Electric Circuit is made up of any thing and every thing which serves to conduct the electric current in its passage—outward and returning—from where it leaves the inner surfaces of the zinc plates in the battery cell to where it comes back again to the outer surfaces of the same plates. ”

“ Economy in the construction of a transmission line points strongly to the use of a single circuit, because this means only one line of poles, usually but one cross-arm for the power wires per pole, the least possible number of pins and insulators, and the smallest amount of labor for the erection of the conductors. In favor of a single circuit there is also the argument of greatest mechanical strength in each conductor, since the single circuit is to have the same weight as that of all the circuits that may be adopted in its place. ”

“ The conducting power of insulated copper wire is such that it practically makes no difference whether one of the wires connected with the pole of a battery is two feet or 2,000 miles in length, and the earth, being a conducting medium, supplies an equal extension from the other pole, so that a closed [60] electric circuit may be established across the Atlantic as easily as within the walls of a laboratory. ”

“ The electric circuit is the most valuable servant that we have to warn us of danger. Dotted about the streets are posts carrying at the top a circular box, which contains a knob. As soon as a fire is observed, anyone may run to such a post, smash the glass screening the knob, and pull out the latter. ”

“ The greater diameter of aluminum over equivalent copper conductors has advantages in transmission with alternating current at very high voltages. At high voltages, say of 40,000 or more, the constant silent loss of energy from one conductor to another of the same circuit through the air tends to become large and even prohibitive in amount. ”

“ The finer the wire, the more its resistance. The large wire that you see on the poles connects directly with this fine relay wire. When the electric circuit is complete, the soft iron bars are magnets; when it is broken, they are not. Thus is produced a backward and a forward movement of the armature which opens and closes a second or local circuit, in which is placed the sounder, having two similar coils, but wound with comparatively coarse wire. ”

“ Well, suppose we had an electric battery giving a certain quantity of current, say five ampères, and we should take a large wire that would offer no resistance to that quantity and put it from one side of the battery to the other, a large current would flow at once and tend to exhaust the battery. This is called a short circuit because there is little or no resistance, and it provides the current with an easy path to escape. Remember this, that electricity always takes the easiest path. It will take as many paths as are offered, but the largest quantity will always take the easiest. ”

“ A short circuit an "easy circuit." You always get a short circuit when you give electricity an easy way to get from one wire to the other. But you get no current unless you give it some way to pass from one wire to the other, thus completing the circuit. Therefore you should always complete the circuit through something which resists the flow of electricity, like an electric lamp, a heater, or an iron. ”

“ On most, if not all, transmission lines the guard wire or wires, if used at all, are placed either above or on a level with the highest power conductors. With one conductor of a three-phase circuit mounted on a pin set in the top of a pole, and the two remaining conductors on a two-pin cross-arm beneath, in the method most frequently adopted for transmission lines of very high voltage, it is obviously impracticable to put guard wires either above or on a level with the power circuits. ”

“ When he thrusts his head under the door and pushes it, it swings in a little, bringing one metal strip against another, which belongs to the primary circuit. This closes that circuit. He will never hear the bell ring, for the electric current which will shock him to death travels 186,000 miles per second, while his sensations travel only sixty miles an hour. ”

“ If I connect by a wire one end or pole of the battery with the other, I, being close at hand, can see a small spark. There is now flowing through that connecting wire what we call an electric current, which passes from one end of the battery through the wire to the other end. When there is very little resistance offered to the passage of the current, there is no [Pg 90] sensible heat developed; but if I sever the wire in the middle and unite the ends by a thin platinum wire, the thin platinum wire introduced into the circuit is first raised to incandescence and then fused. ”

“ The difficulty of operating a lamp on the alternating current circuit lies in the fact that while a current will flow freely through it in one direction, when the current reverses the negative electrode or cathode acts as an electric valve and stops the current, thus breaking the circuit and putting out the light. ”

“ Suppose we treat the machine as a dynamo. Bring the ends of the line wire together, thus, as we say, closing the circuit. By some external force let us cause the armature to rotate and under the influence of the magnetic field it will generate an electric current, part of which will pass through the field and part through the line circuit. We may adjust the relative amount of wire in field and line so that any portion of the current we choose will pass through the field. ”

“ This is particularly true in cases where energy transmitted over a single circuit is distributed for both incandescent lamps and large electric motors, because the starting and operation of such motors causes large fluctuations of amperes and terminal voltage on the transmission circuits. To hold such fluctuations within limits which a sub-station can readily compensate for, it is necessary that the loss in the transmission line be moderate, say often within ten per cent of the total voltage delivered to it at maximum load. ”

“ When the lamp burned out the entire 1200 volts was impressed across this piece of paper, puncturing it and so short circuiting the base terminals. Should one or more lamps go out on a circuit, the increase in current above the normal 3 amperes was prevented by an adjustable resistance, or an extra lot of lamps which could be turned on one at a time, connected to each circuit and located in the power station under the control of the operator. This system disappeared from use with the advent of the constant current transformer. ”

“ The facts described in the paper are commenced by one intended to illustrate future reasoning, by shewing what takes place when a very feeble electric current traverses a metallic circuit, interrupted in one part by a neutral solution, into which the two extremities of the wires forming the circuit are immersed. ”

“ Electric cut-offs are provided in the loop or derived circuits from the main to cut the magazines out of the circuit when such predetermined limit of energy is in reserve, and means are employed to prevent the backward flow of the current toward the source from the power stations supplied from the main or other circuit. ”

“ Division of the Light.—Hitherto a separate pile, or a separate machine, has been necessary for the production of each electric light, and it has been found impossible to place two lamps in one circuit. This is readily understood when we consider the mechanism of the regulating apparatus. In electric lamps the approximation and the separating of tlie carbons are controlled by an electro-magnet, which itself follows the variations of resistance in the circuit produced by changes in the length of the voltaic arc. ”

“ On very long lines the use of numerous circuits composed of rather small conductors would obviously increase the constant expense of inspection and repairs and add materially to uncertainty of the service. Very few, if any, transmission lines of as much as twenty-five miles in length are divided into more than two circuits, and in several instances lines of superlative length have only a single circuit each. ”

“ Fig. 3
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For the party who has electric light in his house there is still an easier solution for the problem of power. If the lighting circuit gives 110 volts he can connect eleven 10-volt lamps in series. These will give 3 cp. each, and the whole set of 11 will take one ampere of current, and cost about the same as a 32-cp. lamp, or 1-1/4 cents per hour. Simply connect the miniature circuit to an Edison plug, and insert in the nearest lamp socket. ”

“ As the circuits under consideration convey thousands of horse-power at 11,000 and 22,000 volts it may be that time-limit devices with circuit-breakers would give good results under less exacting conditions. ”

“ Equipment is available for setting up all types of electric circuits, reactive and non-reactive, for either direct or alternating current power, together with the usual voltmeters, ammeters, and wattmeters, required in the measurement of these circuits.
Several examples of each of the fundamental types of generators and motors for both direct and alternating current, as well as the usual transformers and various special types are conveniently arranged for study and complete tests. ”

“ If it be imperfect, the current will leave the wire and jump to the frame and thence back to the battery without performing the work required of it. When this occurs it is known as a 'short circuit.' Electricity always travels by the easiest path, and, if it can avoid doing any work, it will do so. ”

“ We have seen that when too large a current passes through a wire, the wire becomes heated and may even be melted. Buildings are wired to use certain currents, and if from any cause much more current than the regular amount should suddenly pass through the service wires into the house, [120] the various smaller wires would become overheated, and perhaps melt or start a fire. An accidental short circuit, for example, would so reduce resistance that too much current would suddenly rush through the wires. ”

“ But if we connect these wires to the coils of the induction apparatus and then take hold of the two ends of the secondary coil and break and close the primary circuit we should feel a painful shock at each break and close, although the actual amount of current flowing through the secondary wire is not as great as that which flows through the primary; but the voltage (or electromotive force) is higher, and thus is able to drive what current there is through a conductor of higher resistance, such as the human body. ”