# Electroscope

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

For almost currentless measurement of small electrical voltages, see also electrometer amplifier . A rotation voltmeter (also called a "field mill") can be used to measure the electrical field strength without power.

Pointer electroscope

## Function description

Scheme of an electroscope

The function is described using the example of a pointer electroscope (Braun's electroscope). The two connections ensure that the voltage to be measured is applied between the housing and the vertical rod. The rod itself is conductive. Since it is guided through the housing in an isolated manner, no charges can flow from the rod onto the housing. Since the pointer, which is rotatably attached to the rod, is made of conductive material, it charges itself to the rod with the same name, it is repelled by the rod (and attracted by the housing), the pointer deflects due to this electrostatic force. The pointer moves upwards until the torque generated by gravity compensates that of the electrostatic force. If scaling has been carried out, the scale is labeled with values ​​of the electrical voltage.

Electroscopes measure without current, i. that is, they are based on electrostatics . Ideally, no current flows into the measuring device during the DC voltage measurement that could load the measurement object and thus falsify the measurement. Strictly speaking, however, a current flows briefly at the beginning to charge the device's own capacity. The electroscope therefore remains charged when the voltage is removed. Only when the charge is drained (e.g. by short-circuiting against the case) does the pointer return to its rest position as a result of gravity.

Over time, however, charge is lost due to leakage currents , so that the pointer deflection slowly decreases again even without contact. In the event of a charge reversal to an equally large voltage of opposite polarity, the pointer goes back and deflects again by the same distance. If the mechanical inertia of the pointer is too great, it does not reach its zero position, but only twitches briefly.

Since the instrument works independently of polarity, it is also suitable for displaying alternating voltages. However, due to the capacitance, it forms a reactance and cannot measure without current: the constant charge reversal means that displacement currents flow.

The effects of the electrostatic force and the force of gravity depend non-linearly on the deflection of the pointer. A large deflection up to a deflection of almost 90 ° can only be achieved with large loads.

## Designs

### Pointer electroscope

The simplest design is the pointer electroscope, it is also the most commonly used. After its inventor Karl Ferdinand Braun will also Braun electroscope or Brown MOORISH Electroscope called. It essentially consists of an isolated metal rod to which a metal pointer is attached, the center of gravity of which is below the pivot point. If an electrical charge is applied to this arrangement, the rod and pointer repel and the pointer is deflected. The larger the charge, the greater the pointer deflection.

### Double pointer electroscope

Electroscope with folded foil as display

Sensitive devices often use double pointers instead of a single pointer. As a result, the restoring force (gravity) of the pointer, which is almost balanced like a scale, can be kept very low. This also makes it more sensitive to external influences such as vibrations or air currents. Double-pointer instruments are therefore usually of a relatively massive construction and encapsulated by glass plates.

Double-pointer electroscopes already respond to small, e.g. B. generated by rubbing plastic objects on textiles or fur, electrostatic charges ( static electricity ). If you hold the rubbed plastic object against the electrometer, part of its charge is transferred to the pointer arrangement and a pointer deflection is observed.

### Foil electroscope

This design, also known as the “leaflet electroscope”, consists of a folded strip of gold , aluminum or copper foil , which may be hung over a wire bracket in a vacuum. When the device is charged, the film halves spread apart in a V-shape. This arrangement is also very sensitive, but no scale can be attached to it - the foil strips are too light and flexible and would lie against the scale or be disturbed by it.

The two halves of the film should have a certain minimum distance in the de-energized state so that their inner surfaces do not touch. Otherwise they could stick to one another even when voltage is applied, which is not acceptable in a safety-relevant application.

The rash increases with a smaller film thickness or mass. Apart from the increased electrical capacitance, the film width has no influence on the deflection. The length of the film, on the other hand, mainly influences the shape and thus the visibility of the fold.

Wulf thread electrometers use one or two threads that are lightly tensioned with a bracket and spread when a voltage is applied.

### Potential-free (bipolar) electroscopes

Pointer electroscopes are also available in bipolar, i.e. H. symmetrical (ungrounded) design can be produced, e.g. B. by means of insulated storage of the pointer between two electrodes isolated from ground potential. However, they are less practical because they work on the principle of attraction and the force also increases when you approach them, so that the scale is unfavorably divided. Furthermore, there is more of a risk of flashover ( spark ) from the pointer approaching the electrodes.

#### Bohnenberger design

In a design according to Bohnenberger , voltages can be compared or differential voltages between the electrodes can be verified based on the change in position of a gold plate hanging between two plate electrodes. When there is a voltage difference, a torque arises which deflects the plate from its rest position (parallel to the plates) and aligns its plane in the direction of the electric field lines. The length of the electric field between the plates is shortened. Bohnenberger's device is thus a comparator - a scale cannot be implemented in this device either, as it would interfere with the field.

#### Flutter sheet electroscope

Animation of a flutter blade electroscope

The fluttering leaf electroscope is a variant of the Zamboni - pendulum , is like this to electrostatic commuting. Since this device transports charges like those, it does not work without power and therefore does not actually belong to the electrostatic measuring devices. The flutter blade electroscope is an air- insulated vertical capacitor , between whose plates, which are insulated from earth, a rectangular metal plate stands on its lower edge, electrically insulated. When the plate tilts towards one of the capacitor plates, it takes on its charge and is therefore tilted by the electrostatic field to the other plate, where its charge and direction of movement reverse again.

### Capillary electrometer

As a measuring principle, this design uses the physical property of the surface tension of a mercury column in a capillary tube that is covered with dilute sulfuric acid at the top.

### Voltage balance

The voltage balance , also known as an "absolute electrometer", is a beam balance whose one load is a capacitor plate . The change in the force on the capacitor plate due to the electric field is compared directly with the weight of a known mass.

## Energy balance

Electrometers work mechanically, the pointer deflection means mechanical work. It follows that when the device is in operation, electrical energy must have flowed into the device. Most of the energy is in the charge of the structure (self-capacitance) and is not converted. However, some of it becomes kinetic energy (pointer moves) and potential energy (pointer deflection). While the potential energy is converted back into electrical energy during discharge, the kinetic energy can be converted into heat through inelastic collisions as well as air and bearing friction. Part of the energy can also flow into the plastic deformation of the foils.

The current flow of the charge shift, on the other hand, does not cause any significant losses due to the comparatively low electrical resistance of the pointer and the suspension. The current flow generates a magnetic field, but this does not play a role in the energy balance either.

The energy that is constantly lost even in the steady state is carried away by the leakage currents. These include, on the one hand, currents through the non-ideal insulators (heat) and, on the other hand, charge losses through ionized, detaching and recombining air and water molecules as well as dust particles.

## Historical meaning

Historic gold foil electroscope

The discovery of radium and polonium succeeded Marie and Pierre Curie using a simple electroscope. This does not directly show the ionizing radiation , but the speed of the discharge is accelerated by ionizing radiation and the associated increase in the conductivity of the air. This enables conclusions to be drawn about the radioactivity . This principle is z. B. used in dosimeters .

Measurements of air electricity (field strength in the atmosphere, with or without thunderstorms) and experiments with ultraviolet radiation were also carried out with electrometers.