Electrometer

“ The pointed wire under the prime conductor continuing of the same height, pinch it between the thumb and finger near the top, so as just to conceal the point; then turning the globe, the electrometer will rise and mark the full charge. Slip the fingers down so as to discover about half an inch of the wire, then another half inch, and then another; at every one of these motions discovering more and more of the pointed wire; you will see the electrometer fall quick and proportionably, stopping when you stop. ”

“ Although the electroscope can be used with advantage in special cases, it is limited in its application. The most generally convenient apparatus for measurement of ionization currents through gases is one of the numerous types of quadrant electrometer. With the help of auxiliary capacities, the electrometer can be used to measure currents with accuracy over a wide range, and can be employed for practically every kind of measurement required in radio-activity. ”

“ But if we can make the potential of the electrode of the electrometer very nearly equal to that of the body before making contact, then the uncertainty of the values of and will be of little consequence.
If we know the value of the potential of the body approximately, we may charge the electrode by means of a 'replenisher' or otherwise to this approximate potential, and the next experiment will give a closer approximation. In this way we may measure the potential of a conductor whose capacity is small compared with that of the electrometer. ”

“ If the two sides of the spark gap are connected with the two pairs of quadrants of a very delicate electrometer, the needle of which is connected with one pair of quadrants, there will be a deflection of the electrometer when the detector is struck by electric waves. A very efficient detector is that invented by E. Rutherford (Trans. Roy. Soc. A. 1897, 189, p. 1) ; it consists of a bundle of fine iron wires magnetized to saturation and placed inside a small magnetizing coil, through which the electric waves cause rapidly alternating currents to pass which demagnetize the soft iron. ”

“ A capillary electrometer can be inserted in place of the frog-muscle indicator, and the movements of the electrometer photographed on a sensitized plate moved by clockwork (Willem Einthoven) . Each sound gives rise to a succession of vibrations of the mercury meniscus of the capillary electrometer. ”

“ The methods of measurement previously described depend upon the rate of angular movement of a suspended gold-leaf or of an electrometer needle. The galvanometer can only be employed for measurements with intensely active matter. A need, however, has long been felt for a method in which ordinary ionization currents can be measured by means of a steady deflection of an electrometer needle. This is especially the case, where measurements have to be made with active substances whose activity alters rapidly in the course of a few minutes. ”

“ The whole apparatus is placed in a Leyden jar, of which the inner surface is charged and connected with the attracted disk and guard-ring. The other disk is worked by a micrometer screw and is connected first with the earth and then with the conductor whose potential is to be measured. The difference of readings multiplied by a constant to be determined for each electrometer gives the potential required. ”

“ This insulating power of ice is not effective with electricity of exalted intensity. On touching a diverged gold-leaf electrometer with a wire connected with the platina, whilst the tin case was touched by the hand or another wire, the electrometer was instantly discharged (419.) ”

“ In other words, the electrometer nearly obeyed the theoretical law. Lastly, when the quadrants were 4 mm. or more apart, the deflection increased much more rapidly than the potential, so that a maximum sensibility bordering on instability was obtained. Finally, these observers traced the variation to the fact that the wire supporting the aluminium needle as well as the wire which connects the needle with the sulphuric acid in the Leyden jar in the White pattern of Leyden jar is enclosed in a metallic guard tube to screen the wire from external action. ”

“ To determine the galvanometer constant, substitute a high resistance R, say one megohm, for the line, and shunt the galvanometer with a shunt s. If the deflexion under these circumstances is d and G is the galvanometer resistance, the constant is
Electrometer method. (2) The electrometer method is only applicable to lines of considerable inductive capacity, but is particularly well suited for cable testing. The battery B (fig. 11) is connected through a reversing key K1 to the ends of the resistance slide ab, one end of which is put to earth. ”

“ Iodide of potassium which had been fused and cooled was introduced into the course of the electricity from the machine. There were two pieces, each about a quarter of an inch in thickness, and exposing a surface on each side equal to about half a square inch; these were placed upon platina plates, one connected with the machine and electrometer (419.) , and the other with the discharging train, whilst a fine platina wire connected the two pieces, resting upon them by its two points. On working the electrical machine, it was possible to open the electrometer leaves about two-thirds of an inch. ”

“ Our electrometer, however, was a great improvement upon any previous type, and far removed from the simple pith-ball device used by Franklin. Knowing that an electrified body free to move between two other electrified bodies will always move from the higher to the lower potential. ”

“ Again, if two iron rods are driven into the earth and connected by a copper wire with an electrometer in the circuit, the instrument is almost immediately affected, showing that currents of electricity are running through the ground. ”

“ When the weather is damp, and the electricity pretty strong, the index of an electrometer, after taking a spark from the string, or being presented to the knob of a coated phial, rises surprisingly quick to its former place ”

“ The importance of this investigation resides in the fact that an electrometer of the above pattern can be used as a wattmeter (q.v.) , provided that the deflection of the needle is proportional to the potential difference of the quadrants. This use of the instrument was proposed simultaneously in 1881 by Professors Ayrton and G. F. Fitzgerald and M. A. Potier. Suppose we have an inductive and a non-inductive circuit in series, which is traversed by a periodic current, and that we desire to know the power being absorbed to the inductive circuit. ”

“ The higher the potential of the more sensitive is the instrument. This electrification of , being independent of the electrification to be measured, places this electrometer in the heterostatic class. ”

“ If by means of an electrometer we determine the electric potential at different points of a circuit in which a constant electric current is maintained, we shall find that in any portion of the circuit consisting of a single metal of uniform temperature throughout, the potential at any point exceeds that at any other point farther on in the direction of the current by a quantity depending on the strength of the current and on the nature and dimensions of the intervening portion of the circuit. ”

“ In the investigation of radio-active bodies this arrangement is quite defective; the air being a conductor there is incomplete insulation between the thread and the cylinder, and the inevitable electromotive force of contact between the thread and the cylinder tends to produce a current through the air, and to cause a deflection of the electrometer. We now screen all the wires from the air by placing them inside cylinders filled with paraffin or other insulating material. ”

“ Tension.—When a voltaic battery of 100 pairs of plates has its extremities examined by the ordinary electrometer, it is well known that they are found positive and negative, the gold leaves at the same extremity repelling each other, the gold leaves at different extremities attracting each other, even when half an inch or more of air intervenes. ”

“ Before each movable disk there was a screen, A (Fig. 2) , containing an aperture through which only one sector could, be seen at a time. Finally, before each screen there was a pith-ball electrometer. The two electrometers were connected together by means of a conductor (C) passing under the earth, and which at either of its extremities could be put in communication with either an electric machine or the ground. A lever handle, J, interposed into the circuit a Volta's pistol, F, that served as a call. ”

“ The rod, to be made active, was introduced through an opening in the stopper and exposed in the presence of the emanation for a definite interval. If the decay was to be measured by the α rays, the rod was made the central electrode in a cylindrical vessel such as is shown in Fig. 18. A saturating voltage was applied, and the current between the cylinders measured by an electrometer. If a very active rod is to be tested, a sensitive galvanometer can be employed, but, in such a case, a large voltage is required to produce saturation. ”

“ He had shown, at paragraph 371 and p. 105 of his “Researches,” that as a measure of quantity, a voltaic group of two small wires of platinum and zinc, placed near each other, and immersed in dilute acid for three seconds, yields as much electricity as the electrical battery, charged by thirty turns of a large machine; a fact that was established both by its momentary electro-magnetic effect, and by the amount of its chemical action, but, in order to enable him to establish a principle of definite measurement, he devised a voltameter or volta-electrometer as mentioned at paragraph No. ”

“ He thinks it would be worth while to try, by a 71 very sensible electrometer, whether an insulated muscle, when stimulated into action, would not give indications of the liberation of electric fluid, although he suspects that in man the quantity is probably too small, and too slowly liberated, to be ascertainable. ”

“ When the resistances to be measured are very great, the comparison of the potentials at different points of the system may be made by means of a delicate electrometer, such as the Quadrant Electrometer described in Art. ”

“ There are methods of measuring electrical power by means of electrostatic voltmeters, or of quadrant electrometers adapted for the purpose, which when so employed may be called electrostatic wattmeters. ”

“ The electrification of the "needle" was kept up by enclosing the quadrantal box within an electrified Leyden jar, to the interior coating of which contact is made by a platinum wire, depending from the needle to sulphuric acid contained in the jar. The whole apparatus was enclosed in a conducting case connected to earth. This made its action perfectly definite. Variations of this electrification of the jar were shown225 by an attached attracted-disk electrometer, the principle of which we shall merely indicate. ”

“ The most important improvements in connexion with electrometers are due, however, to Lord Kelvin, who introduced the guard plate and used gravity or the torsion of a wire as a means for evaluating the electrical forces. ”

“ Jeffries paid one hundred guineas to accompany Blanchard on his fifth ascent, which was made from London November 30, 1784. He took with him a thermometer, a barometer, a hygrometer, an electrometer, and a mariner's compass, also several numbered bottles, filled with water and provided with glass stoppers, which were to be emptied and corked up at different heights in the atmosphere. It was arranged to record the observations on ruled paper with a silver pen, because the doctor would not trust a common pen or pencil as liable to accident. ”

“ The action of detectors, etc., depends upon this fact; and, strange to say, the magnetic field about the wire disappears the instant the current ceases to pass. The combination, thus, of a coil of wire and a magnetic needle, properly arranged, makes an instrument with which the presence of electricity can be detected. When the strength of a current is to be measured, or the strengths of two currents are to be compared, the apparatus is called a galvanometer. The method of making these pieces of apparatus will depend upon the strength of current to be tested or measured. ”

“ Eighth Experiment.
When a brass wire is rubbed and brought to the electroscope, the leaves do not move, in consequence of the electricity passing away to the earth through the body as fast as it is generated: it is just like pouring water into a leaky cistern; but if the brass wire is tied to a long stick of sealing-wax, and this latter held in the hand whilst the wire is rubbed with a bit of flannel, then the gold leaves of the electroscope are affected, on account of the insulation of the metal, as every substance which can be rubbed (even fluids, as water) produces electricity. ”

“ The 5th series (1833) is occupied with his electrochemical researches. In the 7th series (1834) he defines a number of new terms, such as electrolyte, electrolysis, anode and cathode, &c., in connexion with electrolytic phenomena, which were immediately adopted into the vocabulary of science. His most important contribution at this date was the invention of the voltameter and his enunciation of the laws of electrolysis. The voltameter provided a means of measuring quantity of electricity, and in the hands of Faraday and his successors became an appliance of fundamental importance. ”

“ Fig. 10.—Electric windmill which operates by the reaction due to the escape of the electric charge from the points.
Electroscopes.—These are instruments for detecting whether a body be electrified or not, and indicating also whether the electrification be positive or negative. The earliest electroscope devised consisted of a stiff straw balanced lightly upon a sharp point; a thin strip of brass or wood, or even a goose quill, balanced upon a sewing needle will serve equally well. Another form of electroscope is the pith ball pendulum, shown in figs. ”