Direct current

“ A direct current, see Fig. 49, is a current which flows steadily in one direction and this can be generated by a battery or a dynamo. An interrupted current, see Fig. 50, is a current that is made and broken a number of times a minute and this is usually done by a vibrator, or interruptor as it is often called. A pulsating current, see Fig. 51, is one whose current strength is varied. One way to produce a pulsating current is to talk into a telephone transmitter which is connected with a battery.
Fig. 49. A STEADY DIRECT CURRENT
Fig. ”

“ Whoever makes a great discovery, such as taking the electric current direct, or dividing its capabilities into useful and necessary directions, or perfects some great mechanism, securing the full beneficence of the current, brings it here and dedicates it to the Commonwealth and its sons and daughters. Its benefits are common to all. ”

“ And how vastly has electricity, whether in the workshop or laboratory, enlarged our conceptions of the forces that thrill space, of the substances, seemingly so simple, that surround us—substances that propound questions of structure and behaviour that silence the acutest investigator. “You ask me,” said a great physicist, “if I have a theory of the universe? Why, I haven't even a theory of magnetism!”
The conventional phrase “conducting a current” is now understood to be mere figure of speech; it is thought that a wire does little else than give direction to electric energy. ”

“ We can understand how an electric current travels through a wire, even if we cannot understand electricity, but how that current can travel through limitless space and yet reach its destination strains the imagination. ”

“ To this conception was soon added the notion of a chemical current, the notion of an electric, magnetic, and anisotropic optical field, intimately connected with the path of the current. And the richer a conception becomes in following and keeping pace with facts, the better adapted it is to anticipate them. ”

“ The current seems to try to get further away from every part of itself. It would appear that the branches are mutually repelled, but this is not the case, as proved by the law that currents flowing in the same direction mutually attract. ”

“ FARADAY’S discovery that, in combinations like those in which a voltaic current was known to produce motion, motion would produce a voltaic current, naturally excited great attention among the scientific men of Europe. ”

“ Disturbance of equilibrium and sympathetic equation constitute the dual power that governs all the varied forms of life and motion which exist terrestrially, of which the electric or magnetic is the prime mover and regulator. All electrical action, no matter of what character, has its sympathetic birth by the intervention of that current of the triune flow, which I call the dominant, with the Polar harmonic current ”

“ We observe how the energy of an alternating current traversing the wire manifests itself--not so much in the wire as in the surrounding space--in the most surprising manner, taking the forms of heat, light, mechanical energy, and, most surprising of all, even chemical affinity. ”

“ That the quantity of heat developed in a second in a wire conveying an electric current is quadrupled on the doubling of the strength of the current, we explain from the doubling of the fall of the potential due to the doubling of the current's intensity, as also from the doubling of the quantity flowing through, in a word, from the quadrupling of the work done. ”

“ If I can stop some of the free energy that is vibrating all around us, I'm going to get either motion or heat out of it, and that's simple. We have gone far enough to know that this little receiver here, gentlemen, will arrest the free current of electricity, force, energy, whatever you care to call it, that's in the air and which can be multiplied and transmitted through the air. ”

“ To find the work done by the current in unit of time in overcoming resistance, and so generating heat, we multiply the components of the current by the corresponding components of the electromotive force. ”

“ If, on the other hand, we make the potential, and the function of flow, then the case will be that of a current in the general direction of , flowing through a sheet in which a number of non-conducting divisions are placed parallel to , extending from the axis of to an indefinite distance in the negative direction. ”

“ For, if every carrier exists naturally for the sake of the thing carried, and if every current or rotation is a current of something carried, there can exist no current either for the sake of itself or for the sake of some other current. Every current must exist for the sake of the Planets, and with a view to their rotation. ”

“ The action simply develops potential difference, and this generates the current where a circuit exists. While we are in the habit of saying that a conductor moved across a field of lines, or vice versa, generates electric current, I think the statement incomplete. The movement only sets up a potential difference, and the power expended in effecting the movement generates C × E. The current is energy less the potential, or the energy expended gives the two effects of potential or pressure and current or rate of movement. ”

“ Potential.—The potential of an electrified body is its degree of electrification. Therefore, it is said that electroscope C mentioned above has a greater potential than electroscope D. The movement of electricity is from a place of greater or high potential to one of lesser or low potential. If two bodies are at the same potential there will be found no movement of electricity between them. A difference of potential between two points connected by a conductor is therefore the necessary condition for an electric current. ”

“ When a current increases, there develops an electromotive force of self-induction which tends to oppose the current; on the contrary, when the current decreases, the electromotive force of self-induction tends to maintain the current. ”

“ After the watch has been placed in position, an alternating current, that is, one whose direction is changing at regular intervals, is sent through the winding, and thus a magnetic field is produced that also changes in direction as the current reverses. ”

“ It appeared that an electric current, which had been thought of as a continuous phenomenon, consists of electrons traveling one way and positive ions traveling the other way; it is no more strictly continuous than the stream of people going up and down an escalator. ”

“ For instance, we cannot directly or consciously generate an electric current, or magnetism, or light, or life; for all these things we depend upon partially explored properties of matter, which we can arrange in a certain way so as to achieve a desired end. ”

“ Professor Ferdinand Braun of Strasburg has conceived the idea of employing a mixed system, in which the earth and the water, which, as we have seen, have often been utilised to conduct a current for transmitting a signal, will serve as a sort of guide to the waves themselves. ”

“ Imagine the whole of the atmosphere divided into two great currents: a hot current continually ascending, and a cold current continually descending. And let the hot current represent the stream of conscious life, the cold current the stream of material things. To complete the simile, conceive that there is a sharp boundary between the two currents, so that atoms of air can never cross to and fro; while yet the conscious atoms in the hot current are aware of the existence of the unconscious atoms in the cold one. ”

“ We have merely shewn that it is possible to conceive the mutual action of electric currents to depend on a particular kind of stress in the surrounding medium, instead of being a direct and immediate action at a distance. ”

“ If there is no electric current through any part of a closed surface, and no electrodes or intrinsic electromotive forces within the surface, there will be no currents within the surface, and the potential will be uniform. ”

“ This distinction of quantity and intensity is very useful [11] , but must be distinctly understood to mean nothing more than this :— The quantity of a current is the amount of electricity which it transmits in unit of time, and is measured by the number of unit currents which it contains. The intensity of a current is its power of overcoming resistance, and is measured by or , where is the resistance of the whole circuit. ”

“ The amount of energy which may thus be passed into the body of a person depends on the frequency and potential of the currents, and by making both of these very great, a vast amount of energy may be passed into the body without causing any discomfort, except perhaps, in the arm, which is traversed by a true conduction current. ”

“ If a number of closed unit currents in contact exist on a surface, then at all points in which two currents are in contact there will be two equal and opposite currents which will produce no effect, but all round the boundary of the surface occupied by the currents there will be a residual current not neutralized by any other ”

“ Any thin sheet of metal or conducting matter bounded on both sides by air or some other non-conducting medium may be treated as a current-sheet, in which the distribution of the current may be expressed by means of a current-function. ”

“ A worker who does not know the difference between a binding post and an alternating current need not expect to progress very far with an invention of, say, an electric block signal system—unless he calls in an expert to help him; but what he should do is to study the principles of electricity and magnetism, learn [59] the various currents that can be used and what apparatus and instruments are needed for utilizing these currents. ”

“ Now capacity for performing work is nothing else than energy, in whatever way it arises, and all energy is the same in kind, however it may differ in form. The energy of an electric current is either of that form which consists in the actual motion of matter, or of that which consists in the capacity for being set in motion, arising from forces acting between bodies placed in certain positions relative to each other. ”

“ Current.
The adjustment, or effects of a continuous attempt at readjustment of
potential difference by a conductor, q. v., connecting two points of
different potential. A charged particle or body placed in a field of
force tends to move toward the oppositely charged end or portion of the
field. If a series of conducting particles or a conducting body are held
so as to be unable to move, then the charge of the field tends, as it
were, to move through it, and a current results. ”

“ If the current is not so interrupted, it cannot be felt at all, which may be proved by lifting up the spring on the spindle of the ordinary kind. The current is an alternating one, and changes its direction throughout the circuit, however extended it may be, at every half revolution. The current must be commutated. If it is required to have a continuous current, use must be made of what is termed a commutator, and I shall endeavour to explain the manner in which it acts as simply as possible. ”

“ If, on the other hand, we adopt the conception of action through a medium, we are led to inquire into the nature of that action in each part of the medium. If we calculate on this hypothesis the total energy residing in the medium, we shall find it equal to the energy due to the electrification of the conductors on the hypothesis of direct action at a distance. ”

“ An electric current manifests itself by heating the wire or conductor, by causing a magnetic field around the conductor245 and by causing chemical changes in a liquid through which it may pass.
Note.—Statics is that branch of mechanics which treats of the forces which keep bodies at rest or in equilibrium. Dynamics treats of bodies in motion. Hence static electricity is electricity at rest. The earth’s great store of electricity is at rest or in equilibrium. ”

“ The equation of the current in a circuit whose resistance is , and whose coefficient of self-induction is , acted on by an external electromotive force , is
When is constant, the solution is of the form
where is the value of the current at the commencement, and is its final value. ”

“ When the rod is set in motion these electrical forces change. Electricity in motion constitutes an electric current. But electric currents give rise to forces of a different type from those due to electricity at rest, viz. magnetic forces. Moreover these forces arising from the motion of electric charges will naturally be of different intensity in the directions along and across the line of motion. ”

“ Surging Character of High-tension Currents.—When a current of electricity is sent through a wire, hundreds of miles in length, the current surges back and forth on the wire many thousands of times a second. Light comes to us from the sun, over 90,000,000 of miles, through the ether. ”

“ When two bodies are near each other, they intercept the current which would flow in the intermediate space if they were not there, and thus receive a tendency towards each other from the pressure of the currents on their farther sides. ”

“ Now if the atoms or particles in the conscious current should be gifted with senses in proportion to their size, they will see and feel an infinitely minute portion both of the ascending current in which they they themselves are placed, and of the descending current they are passing by. ”

“ One face, say A, is acted on by light. The current of response is from B to A, across the plate.
Typical experiment on the electrical effect induced by light.—This subject of the production of an electrical current by the stimulus of light would appear at first sight very complex. But we shall be able to advance naturally to a clear understanding of its most complicated phenomena if we go through a preliminary consideration of an ideally simple case. ”