Electrical resistance

“ With the realization of all that Ohm's Work meant in the department of electricity, it is easy to understand how his name deserves a place in the science for all time. In order permanently to honor his memory, the International Congress of Electricians, which met at Paris in 1881, confirmed the action of the British Association of 1861, by giving the name ohm to the unit of electrical resistance. This is an ideal monument to the great worker. ”

“ If the condenser is now removed, and a resistance coil substituted for it, and adjusted till the steady current through the galvanometer produces the same deflexion as the succession of discharges, and if is the resistance of the whole circuit when this is the case,
We may thus compare the condenser with its commutator in motion to a wire of a certain electrical resistance, and we may make use of the different methods of measuring resistance described in Arts. ”

“ It is to be here observed, that all kinds of matter oppose some resistance to the progress of what is termed the electrical discharge, but the resistance through capacious metallic bodies is comparatively so small as to admit of being neglected under ordinary circumstances; hence it is, that such bodies have been termed conductors of electricity, whilst bodies such as air, glass, &c., which are found to oppose very considerable resistance to electrical action, are placed at the opposite extremity of the scale, and termed non-conductors or insulators. ”

“ It has already been explained that wires offer resistance to an electrical current, and that some of the energy is lost in passing through a wire because of this resistance. Small wires possess more resistance than large ones, and if small wires are to be used, in order to save on the cost of the transmission line, the loss of energy will be greater, necessitating some method of partially reducing or overcoming this fault. ”

“ The internal resistance of a battery can be changed by connecting the cells differently. (See Chap. on Arrangement of Cells.)
Fig. 96.
EXPERIMENT 121. To find what effect the length of a wire has upon its electrical resistance.
Apparatus. A No. 30 German-silver wire, G-S W, a little over two meters long, un-insulated (No. 81) ; the two-fluid cell, 2-F C (Exp. 113) ; galvanoscope, G V (No. 58) ; plate binding-posts, X, Y and Z (No. 83–84–85) ; copper washers (No. 87) .
Arrange as in Fig. 96, so that the current will flow, at first, as shown by the arrow. ”

“ I have alluded to the resistance offered by the conductor of a telegraph-cable to the passage of an electric current, and to the retardation of this current by static induction. The terms retardation and resistance are not considered technically synonymous, but are intended, as electrical terms, to designate two very different forces. The resistance of a wire, as we have seen above, is proportional to its length, and inversely to its diameter. It is overcome by increasing the number of cells in the battery, or, in other words, by increasing the intensity or force of the current. ”

“ Now if every living animal, and every cell within it, be really an electrical machine—a generator of electro-magnetic energy—it is evident that in order to secure and use the power thus produced the apparatus must be insulated from its surroundings, otherwise the electricity would instantly escape back into the earth whence it came. All our electric machines and batteries are thus insulated.
Are animal bodies provided with this electric insulation? They are. The insulatory resistance of the bare human skin varies from 1,000 to 6,000 ohms. ”

“ How to Make a Wheatstone Bridge The amateur experimenter will find many occasions when it is desirable to know the resistance of some of his electrical apparatus. Telephone receivers, telegraph relays, etc., are all graded according to their resistance in ohms. The measurement of resistance in any electrical instrument or circuit is usually accomplished by comparing its resistance with that of some known circuit, such as a coil of wire which has been previously tested. ”

“ The highly positive surface of the glass where it leaves the rubber is more attracted by the negative charge of the rubber than the partially discharged surface which is advancing towards the rubber. The electrical forces therefore act as a resistance to the force employed in turning the machine. The work done in turning the machine is therefore greater than that spent in overcoming ordinary friction and other resistances, and the excess is employed in producing a state of electrification whose energy is equivalent to this excess. ”

“ Series resistance limits the discharge current to such an extent that an arrester with series resistance fails to protect against destructive rises of voltage when the conditions are severest. Graded shunt resistance responds to all frequencies and opens a discharge path for excessive voltage when the frequencies are high as well as when they are low. Its further effect in withholding the line current from the gaps after the relieving discharge has occurred, is to aid the non-arcing quality of the metal cylinders in quickly suppressing the arc that follows a discharge. ”

“ Should anything happen to interrupt the current while the motor M is running, E will lose its magnetism and A will be released, and will fly over to the off position. E is usually shunted by a small resistance R, so that only a portion of the main current flows through it. This device constitutes the no voltage release, and ensures that all the resistance is in circuit every time the motor is started. ”

“ Such
counter-electro-motive force may be treated as a spurious resistance and
such a value in ohms assigned to it as would correspond to its proper
effect.
468 STANDARD ELECTRICAL DICTIONARY.
In its effect on opposing a current and in resisting its formation it
differs from true resistance. The latter in diminishing current strength
absorbs energy and develops heat; spurious resistance opposes and
diminishes a current without absorption of energy or production of heat. ”

“ On connecting
the binding posts of the thin wire coil to any two parts of the circuit
its resistance is at once given by the deflection of the needle.
When no current is passing the needle rests in any position. A current
in the thick coil brings it to zero. A current simultaneously passing
through the thin high resistance coil brings about the deflection.
The instrument is a commercial rather than a scientific one.
Ohm's Law.
The fundamental law expressing the relations between current,
electro-motive force and resistance in an active electric circuit. ”

“ There is no part of electrical research in which more numerous or more accurate experiments have been made than in the determination of the resistance of metals. It is of the utmost importance in the electric telegraph that the metal of which the wires are made should have the smallest attainable resistance. ”

“ The whole is so adjusted that the
variations in resistance maintain a constant amperage. The resistance is
in the exciting circuit of the dynamo.
In Brush's regulator, which is purely mechanical, a series dynamo is
made to give a constant current by introducing across the field magnets
a shunt of variable resistance, whose resistance is changed by an
electro-magnet, whose coils are in circuit with the main current. Carbon
resistance discs are used which the electro-magnet by its attraction for
its armature, presses with varying intensity. ”

“ It tends to cause shedding of the active material, buckling of plates, loss of capacity, increase of resistance and consequent reduction of efficiency, and increase of temperature with 957 flow of current. A sufficient amount of lead peroxide and sponge lead must be retained on the plates to reduce this resistance, otherwise the charging current cannot flow through the active material and regenerate the battery. ”

“ Let us say that instead of separating these two mains by air we separated them by the human body—that a man took hold of the bare wires, one in each hand. The resistance of the human body varies from 5,000 to 10,000 ohms. In that case C (amperes) equals 110/5,000, or 110/10,000—about 1/50th, or 1/100th of an ampere. This illustrates why an electric current of 110 volts pressure is not fatal to human beings, under ordinary circumstances. ”

“ When electric radiation is absorbed by the sensitive surface, there is a sudden diminution of the resistance, and the galvanometer in circuit is violently deflected. By adjusting the electromotive force of the circuit the sensitiveness of the receiver may be increased to any extent desirable. ”

“ As a resistance in series with an arrester cuts down the generator current that can follow a lightning discharge, such a resistance also decreases the number of air-gaps necessary to give an arrester power to destroy arcs on a particular circuit. ”

“ Electricity when confined to substances resisting its progress, as air, glass, dry wood, stones, &c., exerts a terribly explosive power.
3. But when confined to bodies, such as metals, offering small resistance, its violent expansion or disruptive action is greatly reduced or avoided altogether, and becomes a continuous current comparatively quiescent. But if body be small, as wire, it may be heated or fused. Resistance is so small that a shock has traversed copper wire at the rate of 576,000 miles a second; resistance increases with length and diminishes with area of section of conductor. ”

“ Now, the electricity which is constantly being made fills the wires and acts as a pressure to force the current through the wires back to the dynamo, and, if we put no resistance or opposition in the way, it would have a very easy path to travel in and would do no work at all. The wires leading to an electric lamp should have very little resistance, not sufficient to require any work from the current in passing through. ”

“ Now when the current from a given battery or source of electricity is made to pass through wires of different resistances, the strength of the current which will pass through these wires can be made a measure of their resistances; and therefore, when the current from a particular battery is sent through the conductor of the cable or to test the insulator, and in each case measured by the galvanometer, and compared with the current from the same source which will flow through the units of comparison, the copper resistance and insulation resistance can be obtained. ”

“ If a wire were too small for the volts pressure and ampères of current of electricity the resistance of such wire would be overcome, and it would become red-hot and perhaps melt. Electricians are therefore very careful to calculate the resistance of the wires they use before putting them up, especially when they are for electric lighting, in [13] order to make allowances for the ampères of current to flow through them, so that but little of the electricity will be turned into heat and thus rendered useless for their purpose. ”

“ It should be observed that the phenomenon of suddenly increased conductivity occurs, even if the sensitive substance is not in circuit with a battery at the time it is influenced. Thus, the metallic filings, after having been placed in circuit with a Daniell cell, and its high resistance observed, may then be completely insulated and submitted in this condition to the action of a distant spark, or of a charged rod, or of induced currents. ”

“ If the skin and clothes be wet, the resistance to the current is lessened and it passes more readily into the body. In the same way, if a person stands in close relation to a good conductor and places his hand on one wire of a high-tension electric circuit, he will receive a much more severe shock than if not connected with such conductor. ”

“ For this purpose they invariably use a pulsating or alternating current and reduce the resistance by using metal handles and wet sponges for contact with the skin, but even so a very small amount of current passes. The moderate twitching of the muscles seems to be the end sought.
Men who are supposed to be killed by electric shocks often die from other causes. ”

“ The resistance of wooden pins and cross-arm may be so small compared with that of the air and insulator surfaces that complete the path through them from wire to wire of a circuit, that the effect of these wooden parts in checking the flow of current between conductors is relatively unimportant, and yet the resistances of these pins and the cross-arm may affect their lasting qualities. ”

“ The more wire there is in circuit, the higher will be the resistance and the greater the drop caused by a given current. If we lengthen the arc, the current will be somewhat decreased and the drop in voltage over the resistance will be less, thus allowing a rise in the voltage at the terminals of the lamp. The energy lost in resistances takes the form of heat and all resistances, used for the control of arc lamps, give off much heat and must be located in safe places. The heating also makes them objectionable in small operating rooms in summer, but somewhat welcome in winter. ”

“ As the current also passes through a resistance coil, the moment the ends of the electrodes are separated too great a distance, the resistance prevents a flow of the normal amount of current, and the armature is compelled to reduce its pull. The effect is to cause the two electrodes to again approach each other, and in doing so the arc becomes brighter. ”

“ So, instead of using a second wire, we use the earth to carry back the electricity to the battery, because the earth is a better conductor even than wire. Although a quantity of ground equal in size to the wire would offer thousands of times greater resistance than the wire, yet, owing to the great body [26] of our earth, its total resistance is even less than any telegraph wire used. ”