Electroscope

An electroscope is a device for the detection of electrical charges and voltages. Its operation is based on the attraction and repulsion of electric charges and it is one of the electrostatic voltage measuring devices. An electroscope with calibrated scale is also called electrometer. With it, electric charges and voltages may be not only detected, but also measured. The first electroscope at a very early design, consisting of a pivoted needle on a tip, the so-called versorium, was developed around 1600 by William Gilbert.

For nearly current-free measuring small electrical voltages see also electrometer amplifier. For power-free measurement of the electric field strength (also called field mill ) may be used a rotary voltmeter.

Functional Description

The function is described using the example of a Zeigerelektroskopes ( Brown cal electroscope ). The voltage to be measured by means of the connecting terminals between the housing and the vertical rod, which carries the down link. The rod is insulated through the housing. The rod and rotatably attached to him pointers are conductive and therefore invite you to the same name, so that the pointer deflects. He is repelled by the rod and tightened from the housing.

The pointer rises up on the scale until the torque produced by the force of gravity and the field produced by the electrostatic force have equalized. The scale is divided into values ​​of the electrical voltage.

Electroscopes measure de-energized, that is, they are based on electrostatics. In the meter, no current flows, the load on the DUT and could distort the measurement while the DC voltage measurement in the ideal case. However, strictly speaking, a current flows at the start of short to charge the intrinsic capacitance of the equipment. Upon removal of the voltage, the electroscope is therefore loaded. Only when one creates a flow of the charge (eg by shorting against the case ), the pointer returns by gravity back into the rest position.

With time, however, charge is lost due to leakage currents, so that the deflection of the pointer goes back slowly even without contact. Case of transhipment to an equal voltage of opposite polarity, the pointer goes back and strikes again equidistant from. If the mechanical inertia of the pointer is too large, it does not reach its zero position, but just shrugs short.

As the instrument of the polarity operates independently, it is also suitable for the display of alternating voltages. However, it forms by the capacity of a reactance and can not measure energized: Due to the constant transhipment displacement currents to flow.

The effect of both the electrostatic and the gravitational force does not depend linearly on the deflection of the pointer. A large rash to near 90 ° deflection is reached only for large loads.

Designs

Zeigerelektroskop

The simplest design is the Zeigerelektroskop, which is used most frequently. According to its inventor Karl Ferdinand Braun it is also called brown or brown electroscope electroscope cal. It consists essentially of an isolated prepared metal rod, to which a metal link is mounted, the center of gravity below the pivot point. When an electric charge is applied to this arrangement to staff and hands repel and the pointer is deflected. The greater the charge, the greater is the pointer steering.

Double pointer electroscope

Sensitive devices use instead of a single pointer often double pointer. This enables the restoring force ( gravity ) of a similar scale almost balanced pointer are kept very low. Thus, it is also sensitive to external influences such as vibrations or air currents. Double pointer instruments are therefore usually built relatively solid and encapsulated by glass plates.

Double Pointer electroscopes react to even small, eg by rubbing of plastic articles of textiles or fur generated electrostatic charges ( static electricity ). If you hold the grated plastic object to the electrometer, a part of its load is transferred to the pointer array and a pointer deflection is observed.

Foil electroscope

This also Blättchenelektroskop said design is made of a folded gold, aluminum or copper foil strip is hung in a vacuum, if necessary, by a wire bracket. If the device is charged, the foil halves spread apart v -shaped. This arrangement is also very sensitive, but can affix any scale - the film strips are light and flexible and would add to the scale or be bothered by it.

The two halves of the film should have the power off a certain minimum distance, so do not touch their inner surfaces. Otherwise they might stick to one another even when voltage is applied, which is not acceptable in a safety-related application.

The rash increases at lower film thickness or mass. The film width apart from the increased electrical capacity no effect on the rash. The film length, however, mainly influenced the shape and thus the visibility of the unfolding.

Thread electrometer

Thread electrometer by Wulf use one or two with a strap easily attached rope, which is spread when a voltage.

Erdpotentialfreie (bipolar ) electroscopes

Pointer electroscopes are bipolar, that is symmetric ( ungrounded ) design to produce, for example, by means of insulated storage of the pointer between two insulated electrodes compared to ground potential. However, they are impractical, as they work on the principle of attraction and the power on approach also increases, so that the scale is divided unfavorable. Furthermore, there is a greater risk of a rollover (spark ) through which the electrodes approaching pointer.

Design by beans Berger

In a design by Berger beans can be based on the change in position of a hanging between two plate electrodes gold plate compare voltages and demonstrate differential voltages between the electrodes. At a voltage difference, a torque which deflects the plate from its rest position ( parallel to the plates ) and aligns with its plane in the direction of the electric field lines is formed. The length of the electric field between the plates is reduced. Beans Bergers device is therefore a comparator - a scale is also in this device is not feasible, as it would disturb the field.

Flatter leaf electroscope

The fluttering leaf electroscope is a variant of the Zamboni pendulum, it is like this to the electrostatic oscillation. Since this device is transported as those charges, it does not work without power and therefore does not belong actually to electrostatic measuring instruments. When flutter leaf electroscope is an insulated air with vertical condenser, a rectangular metal plate electrically isolated stands on its lower edge between the insulated against ground plates. If the plate tilts to one of the capacitor plates, it assumes its charge and is therefore tilted back by the electrostatic field to the other plate, where its charge and its direction of motion back again.

Capillary electrometer

This design uses as a measurement principle, the physical property of the surface tension of a mercury in a capillary tube, which is covered at the top with diluted sulfuric acid. → capillary electrometer

Energy balance

Electric meters operate mechanically, the pointer deflection means mechanical work. It follows that, during operation of the device of electrical energy must be flown into the device. Most of the energy is in the charge of the structure ( the self-capacitance ), and is not converted. Part of it is, however, to kinetic energy and potential energy (pointer moves ) ( pointer deflection ). While the potential energy is converted back into electrical energy during discharge, the kinetic energy can be transformed by inelastic collisions, air and bearing friction into heat. Part of the energy to flow in the plastic deformation of the sheets.

The current flow of the charge displacement caused due to the relatively low electrical resistance of the pointer and the suspension on the other hand, no significant losses. The current flow causes a magnetic field - however, this does not matter for the energy balance.

Constantly lost in continuous steady-state energy is discharged through the leakage currents. These include on the one hand flows through the non-ideal insulators (heat) and on the other hand charge losses by ionized, receding and recombining air and water molecules and dust particles.

Historical Significance

The discovery of radium and polonium, Marie and Pierre Curie succeeded with the help of a simple electroscope. This does not directly ionizing radiation, the rate of discharge, however, is accelerated by the ionizing radiation and the concomitant increase in the conductivity of the air. Thus conclusions about the radioactivity is possible. This principle is used, for example, in dosimetry.

Also, measurements of atmospheric electricity ( field strength in the atmosphere, with or without lightning) as well as experiments with ultraviolet radiation were performed with electric meters.