Galvanometer

“ The tube must be tapped between each experiment, and the deflection of the galvanometer brought back to about its original value. This detector is marvellously delicate, but not metrical, the change produced in the resistance depending upon so many things besides the intensity of the waves that the magnitude of the galvanometer deflection is to some extent a matter of chance. Instead of the iron turnings we may use two iron wires, one resting on the other; the resistance of this contact will be altered by the incidence of the waves. ”

“ The Einthoven string galvanometer is another form of sensitive instrument for the measurement of small direct currents. It consists of a fine wire or silvered quartz fibre stretched in a strong magnetic field. When a current passes through the wire it is displaced across the field and the displacement is observed with a microscope.
For the measurement of large currents a “tangent galvanometer” is employed (fig. 3) . Two fixed circular coils are placed apart at a distance equal to the radius of either coil, so that a current passing through them creates in the central Tangent galvanometer. ”

“ Dolbear observes that if a galvanometer is placed between the terminals of a circuit of homogeneous iron wire and heat is applied, no electric effect will be observed; but if the structure of the wire is altered by alternate bending or twisting into a helix, then the galvanometer will indicate a current. The professor employs a helix connected with a battery, and surrounding a portion of the wire in circuit with the galvanometer. The current in the helix magnetizes the circuit wire inclosed, and the galvanometer exhibits the presence of electricity. ”

“ It is a comforting theory to electro-therapeutists that a galvanometer will enable them to administer their doses of electricity with as much exactitude as we daily prescribe so many grains, or so many minims of ordinary medicines; but, like some other theories which save us much trouble, when adopted as theories only, it fails us in practice (at least according to my experience) , and chiefly so, because a galvanometer can be usefully employed only when it is included in the circuit of a continuous current, as, e.g., in aneurismal electro-puncture ”

“ However this may be, certainly the phenomenon occurs on a small scale, for if the pair of knobs or points placed as a shunt to protect a galvanometer or other telegraphic instrument from lightning (or what is easier experimentally and essentially the same thing, from a Leyden jar discharge) be set too close together, the galvanometer will be found to be short-circuited after a spark, and the knobs will be found, both by mechanical and electrical tests, to be feebly united at a single point. ”

“ The galvanometer, battery and coil to be tested are connected in series as shown, and then the unconnected terminal of the galvanometer is brought into contact with the shaft. If then some portion of the insulation of the wire has been abraded or destroyed, thus bringing the bare wire into contact with the metal core, as at A in the figure, the needle of the galvanometer will be deflected since a closed circuit is formed through the core and wire. If the insulation be perfect, the needle will not be deflected. ”

“ This may be shown by keeping the galvanometer circuit open during the application of vibration, and completing it at various short intervals after the cessation, when a persisting electrical effect diminishing rapidly with time will be observed. When the wire is brought to the normal condition, successive responses to uniform stimuli are, in the case of metals which, like tin, show no fatigue, exactly the same. I usually interpose a high external resistance, varying from 1 to 5 megohms, so that the galvanometer deflections may be proportional to the electromotive variations ”

“ The galvanometer, as was said before, is also adjusted by means of a shunt, so that a deflection of one division is a convenient small fraction of an ampere. The absolute values of the E. M. F. and current being known, the resistance of the sensitive substance at any point of the curve can easily be determined. ”

“ Fig. 13. the lower remain cool, we get a strong current going from the bismuth to the antimony across the heated junctions, and we may pass the current so produced round the wire of a galvanometer, and thus, by increasing the number of our junctions, and also by using a very delicate galvanometer, we may get a very perceptible effect for the smallest heating of the upper junctions. This arrangement is called the thermopile, and, in conjunction with the reflecting galvanometer, it affords the most delicate means known for detecting small quantities of heat. ”

“ The galvanometer kick is clearly an effect due to the uniting metals, but it has not yet been fully elucidated; it seems to have been first observed by Mr. Stroh in his excellent researches on microphonic action, related in the Journal of the Society of Telegraph Engineers, 1883 and 1887, and it may possibly be thermo-electric, as Prof. Hughes, who also observed it, thinks likely; but it may be electro-chemical, or it may be connected with an effect observed later by FitzGerald in his galvanometer mode of detecting Hertzian waves, which he published at the Royal Institution in 1890. ”

“ 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. ”

“ Even in the other circuits, magnetic and atmospheric influences play some part, and when their contribution is important, the galvanometer deflection has an uncertain value. What a galvanometer records when traversed by a suddenly varying current depends on other things than its mere resistance. ”

“ It is easy to produce considerable sensitiveness in the galvanometer, but for practical purposes it must always be controlled by the condition that the zero remains fixed, that is to say, the galvanometer needle or coil must come back to exactly the same position when no current is passing through the instrument. ”

“ With a mirror galvanometer it is better to move the scale, or turn the galvanometer round on its axis to restore the correct zero, rather than to twist the coil back; but with a fixed scale and pointer the only remedy is to turn the coil bodily round. In a single-pivot indicator constantly used in the author’s laboratory, the creep amounted to a movement of the end of the pointer through an angle of 2 degrees in the first few months, since when, after the lapse of several years, no further alteration has occurred. ”

“ When the current is steady it is divided between the wire and the galvanometer circuit, and produces a certain permanent deflexion of the galvanometer. If the coil is now removed quickly from and placed in a position in which the coefficient of mutual induction between and is zero (Art. 538) , a current of induction is produced in both circuits, and the galvanometer needle receives an impulse which produces a certain transient deflexion. ”

“ If a coherer is shut up in a complete metallic enclosure, waves cannot get at it, but if wires are led from it to an outside ordinary [Pg 35] galvanometer, it remains nearly as sensitive as it was before (nearly, not quite) , for the circuit picks up the waves and they run along the insulated wires into the closed box. To screen it effectively, it is necessary to enclose battery and galvanometer and every bit of wire connection; the only thing that may be left outside is the needle of the galvanometer. ”

“ Since the function of the particular galvanometer is simply to show a deflection when a current is passing, and to mark zero when there is none, a coil of thick wire with a pocket compass in the center will do all that is required, but care must be taken to remove it sufficiently far away from the disturbing effects of the armature magnetism. ”

“ In this way the wires were similarly situated with respect to the primary circuit, but if the electromotive force were stronger in the one wire than in the other it would produce a current which would be indicated by the galvanometer. He found, however, that such a combination may be exposed to the most powerful electromotive forces due to induction without the galvanometer being affected. He also found that whether the two branches of the compound conductor consisted of two metals, or of a metal and an electrolyte, the galvanometer was not affected [6] . ”

“ The galvanometer is only required to be sensitive enough to detect the existence and direction of a current, without in any way determining its value or comparing its value with that of another current. ”

“ A thin magnetic needle is held in a light, paper sling at F, suspended by a fine, vertical fiber. (From Annales de Chimie et de Physique.)
Cumming considers his straight-wire calibrated “galvanometer” to be a device for “measuring” galvanic electricity; on the other hand, his multiple-loop “galvanoscopes” are for “discovering” galvanic electricity. With the multiplier instrument, he found galvanic effects (i.e., needle deflections) using copper and zinc electrodes with several acids not previously known to create galvanic action. ”

“ The pile being lodged at the bottom of a metallic tube blackened inside, the rays of nonluminous heat emitted from the vessel in a direction oblique to the axis cannot reach the thermoscopic body, and the index of the galvanometer remains perfectly at rest. Matters being now in this state, I take a prism of rock salt and fix it at the mouth of the tube with its axis placed vertically and its refractive angle turned towards the angle formed by a line drawn from the source to the extremity of the tube. (See Plate I. fig. 2.) A considerable deviation is immediately perceived in the galvanometer. ”

“ The dial is graduated to single degrees, and the needle is suspended by a very light cocoon fibre. The whole instrument can be turned about its vertical axis, and a quadrant graduated in degrees upon the base allows the amount of rotation to be accurately measured, and the laws of the sine galvanometer investigated. The instrument is wound to measure .25 ampere to 8 amperes.
Ques. What is the objection to the scale with tangent values?
Ans. It is more difficult to divide an arc into tangent lines with accuracy than into equal degrees.
Ques. What disadvantage has the tangent galvanometer? ”

“ A Wood Mandrel for Attaching Regular Sheets of Sandpaper to Make a Lathe Sander
[389]
How to Construct a Simple Galvanometer
A galvanometer is an instrument used to detect the presence of an electrical current in a circuit or to measure the value of the current in amperes. The operation of practically all galvanometers is based upon the same principle, and they differ chiefly in mechanical construction and the relative arrangement of their different parts. ”

“ The galvanometer having been devised, the next step, indispensable for its use, was to frame a standard by which electrical energy might be measured, and to invent terms by which the amount of such energy could be expressed. It is well known that in order to be able to measure the dimensions of any material object, standard units are required. ”

“ For this purpose I used a galvanometer.
A cell was made, in the way previously described, with two specimens of magnesium powder. Owing to some differences in the two portions there was an initial P. D. of 0.042 V. between the two electrodes. The E. M. F. of the cell was balanced by the potentiometer method, a sensitive galvanometer (with an interposed high resistance) being used as the detector of electromotive variation. Fig. 39 (a) shows the deviation from the balanced position by radiation which nearly reduced the potential difference to half its original value. ”

“ When a mere wire, connected with the galvanometer so as to form a complete circuit, was passed through between the poles, the galvanometer was affected; and upon moving the wire to and fro, so as to make the alternate impulses produced correspond with the vibrations of the needle, the latter could be increased to 20° or 30° on each side the magnetic meridian.
110. Upon connecting the ends of a plate of metal with the galvanometer wires, and then carrying it between the poles from end to end (as in fig. 23.) , in either direction, no effect whatever was produced upon the galvanometer. ”