Resistor

“ If a current be passed through two resistances in series by means of an applied electromotive force, the electric potential falls from one end of the resistances to the other, and, if we apply Ohm’s law to each resistance in succession, we see that, since for each of them E = CR, and C the current is the same through both, E the electromotive force or fall of potential between the ends of each resistance must be proportional to the resistance between them. ”

“ The best comparison to make is that of turning a fan in a barrel of treacle: the more quickly the fan is turned the greater will be the resistance to its motion.
The very fact of revolving the armature, which is nothing more than a specially-wound coil of wire close to the poles of a magnet, produces a current of electricity in the coil; and the faster this coil is turned the greater will be the pressure of the current produced. The quantity produced is dependent upon the resistance of the circuit. ”

“ Suppose such a sphere to be charged and discharged rapidly with electricity from any source, such as a battery. It would take electricity from the source at a certain rate, and would in fact act like a resistance in permitting the passage through it or by it of a certain quantity of electricity per unit of time. If K is the capacity and n is the number of discharges per second, then nK is a quantity of the dimensions of an electric conductivity, or of the reciprocal of a resistance. ”

“ 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. ”

“ When the conductors are in series the combined resistance is the sum of the several resistances. Thus in an electric-bell circuit if the battery has a resistance of 1 ohm, the bell of 2 ohms, and the wire 1 ohm, the total resistance in the circuit is 4 ohms. When conductors are in parallel the combined resistance is always less than the separate resistances. Just as a crowd of people meets less resistance in leaving a building through several exits, so electricity [Pg 300] finds less resistance in moving from one point to another along several parallel lines, than along one of the lines. ”

“ Unit electromotive force (volt) through unit resistance (ohm) gives unit of current (ampère) . It is not the intention to treat the subject mathematically, but I will give you a simple formula for finding the amount of current if you know the resistance and the voltage. The electromotive force divided by the resistance gives the current. C = E / R or current (ampères) equals electromotive force (volts) divided by the resistance (ohms) .
But still further: One ampère of current having one volt pressure will develop one watt of power, which is equal to 1/746 of a horse-power. ”

“ 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. ”

“ For maximum torque obviously both resistance and reactance should be equal and as small as possible. Where a small primary current in starting is of considerable importance, this extra resistance is frequently introduced at starting and cut out afterwards, particularly in cases where large torque is necessary. If great starting torque is not necessary, the primary electromotive force is often diminished by inductive resistances, or a change in the connexions of the transformer from which the motor is fed. ”

“ I observed the return to original resistance in the following manner:—The sensitive substance was placed at K (Fig. 53) , and formed part of a circuit which included a Daniell cell and galvanometer. At first no current passes. Sparks are then caused at S, and the needle of the galvanometer is permanently deflected. On smartly tapping the table supporting the ebonite cap in which the sensitive substance is contained, the original condition is completely restored. When the electric action has been of a powerful character, violent blows are necessary. ”

“ If there be a difference of pressure at the ends of the conductor of one volt, and we find that there is half an ampere of current flowing, we know that the resistance is two ohms. If we have a pressure of two volts maintained between two points, and we connect those two points by a wire, and find that two amperes of current flow through it, we know the resistance of the wire is one ohm. ”

“ According to the received opinions we have here a current of fluid moving uniformly in conducting circuits, which oppose a resistance to the current which has to be overcome by the application of an electro-motive force at some part of the circuit. On account of this resistance to the motion of the fluid the pressure must be different at different points in the circuit. This pressure. which is commonly called electrical tension, is found to be physically identical with the potential in statical electricity, and thus we have the means of connecting the two sets of phenomena. ”

“ But if the object be to make a battery of the least possible resistance, the cells are joined in parallel, that is, all the positive poles are connected together and all the negative poles connected together, thus forming one large cell. If the object be to obtain the largest possible current, then the cells are so arranged as to make the internal and external resistances of the circuit equal. ”

“ But when the electric resistance of a properly shaped conductor of a properly chosen material has been once determined, it is found that it always remains the same for the same temperature, so that the conductor may be used as a standard of resistance, with which that of other conductors can be compared, and the comparison of two resistances is an operation which admits of extreme accuracy. ”

“ Unit electromotive force is produced in a closed circuit if any unit of its length is held in the manner, and moved in the direction and with the velocity, described in the last section. Unit resistance is the resistance which, when acted on by unit electromotive force, transmits unit current. Unit capacity is the capacity of a body which requires unit quantity of electricity to raise its potential by unity. ”

“ In order now to find the resistance which the electric current suffers in its passage through the different wires, I first reduce their lengths all to one diagonal, and indeed to that of the wire of the multiplier, on the principle that two wires of the same metal offer then the same resistance to conduction when their lengths are in the same proportion as their diagonals (See Ohm's Galvanic Chain) . ”

“ Experiment 70. Turn on the switch that lets the electricity flow through the long resistance wire that passes around the porcelain posts. Watch the wire.
The resistance wire you are using is an alloy, a mixture of metals that will resist electricity much more than ordinary metals will. This is the same kind of [pg 232] wire that is used in electric irons and toasters and heaters. It has so great a resistance to the electricity that it is heated red hot, or almost white hot, by the electricity passing through it. ”

“ In the Absolute Methods the resistivity is determined without reference to any other substance, but with reference only to the fundamental standards of length, mass and time. Immense labour has been expended in investigations concerned with the production of a standard of resistance and its evaluation in absolute measure. In some cases the absolute standard is constructed by filling a carefully-calibrated tube of glass with mercury, in order to realize in a material form the official definition of the ohm ”

“ C is current in amperes, E electromotive force in volts, and R resistance in ohms.
Thus a wire with a resistance of 50 ohms would pass 2 amperes with an electromotive force of 100 volts. To find resistance when other two factors are known, the formula is
R = E/C.
In selecting a battery for work, regard must be made to the current required, and its period of flow. ”

“ In motors in which this large starting current is objectionable, it may be reduced very greatly by interpolating resistances in the secondary circuits at starting, the effect of these being to diminish the lag in the secondary circuit and to decrease the demand for primary current. A certain critical value of this resistance gives a maximum torque per ampere in the primary circuit with a given motor, being approximately that total secondary resistance which equals the secondary reactance. ”

“ When an electric current is passed through an electrolyte bounded by metal electrodes, the accumulation of the ions at the electrodes produces the phenomenon called Polarization, which consists in an electromotive force acting in the opposite direction to the current, and producing an apparent increase of the resistance.
When a continuous current is employed, the resistance appears to increase rapidly from the commencement of the current, and at last reaches a value nearly constant. ”

“ The resistance which rarefied gas opposes to an electric current in a magnetic field is greatest in the direction normal to the magnetic lines. The changes of resistance effected by any kind of light in a vacuum tube with alkali metal cathode can be measured galvanometrically. (A Daniell cell gives 100 divisions on a Rosenthal galvanometer when coupled up through such an illuminated tube, each division meaning about 10−10· ampere.) ”

“ 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. ”

“ As a unit of resistance, the International Ohm, which is based upon the ohm equal to 109 units of resistance of the C.G.S. system of electromagnetic units, and is represented by the resistance offered to an unvarying electric current by a column of mercury at the temperature of melting ice 14·4521 grammes in mass, of a constant cross-sectional area and of the length of 106·3 cm. ”

“ In making the experiment we may either begin by adjusting the resistances so that which is the condition that there may be no permanent current, and then adjust the distance between the coils till the galvanometer ceases to indicate a transient current on making and breaking the battery connexion; or, if this distance is not capable of adjustment, we may get rid of the transient current by altering the resistances and in such a way that the ratio of to remains constant. ”

“ If we want a very strong current of great volume, so to speak, for electric light or power, and use a galvanic battery, we should have to have cells of large surface and lower resistance both inside and outside the cells. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”