Electrophore

Historic electrophore, around 1840

An electrophore is a form of historical influence machine and serves to separate electrical charges and to generate high electrical voltages with the help of the influence . The term electrophore is derived from the Greek electron = amber (as a prototype of the carrier of static electricity ) and pherein = to carry.

These and similar types of charge separation and the generation of an electrical voltage were the basis of early electricity research in the 18th century, as it was largely carried out by Alessandro Volta . A usable electrophore was developed by him in 1775. In the same century, the Göttingen scholar Georg Christoph Lichtenberg built large electrophores with a diameter of up to 2.5 m, which enabled arcing over 70 centimeters.

construction

Electrophore in use

An electrophore consists of two parts: a metal plate with an insulated handle, comparable to the plate of a capacitor , and a so-called cake, which is electrically non-conductive and consists of a mixture of resin , sealing wax and shellac . This cake is on a grounded, metal base plate, as shown in the adjacent figure.

How to use

Charge distribution on the electrophore

The cake, grounded on the back, is rubbed with a fur; a cat fur was common . This creates static electricity on the cake in the form of excess negative charges. If the electrically neutral metal plate is placed at a small distance above the cake, the influence of the electric field of the cake charges causes the charge carriers in the metal plate to be shifted without the cake's charge being dissipated. On the side of the metal plate facing the cake there is an accumulation of positive charge carriers (or a lack of negatively charged electrons ), on the opposite side of the metal plate there is an accumulation of negative charge carriers. If the metal plate is now grounded on its side facing away from the cake while it is lying over the charged cake, for example by touching it with the hand, the excess electrons can flow away from this side of the plate. Overall, the metal plate is no longer electrically neutral. It carries an excess of positive charges, which are in equilibrium with the stationary negative charges in the cake. If the earth connection to the surface of the metal plate is now removed by z. B. withdraws the hand, and if the metal plate is only lifted from the cake by its insulated handle, an increasing electrical voltage builds up between the metal plate and earth potential as the distance increases . Their height can be determined according to the relationship: ${\displaystyle U}$

${\displaystyle U={\frac {Q}{C}}}$

Calculate from the amount of charge and the capacity of the metal plate-earth arrangement. Since there is no current flow and therefore no change in the electrical charge , but at the same time the capacity of the arrangement decreases due to the lifting of the metal plate, the electrical voltage must increase by the factor by which the electrical capacity decreases. This allows very high electrical voltages in the range of a few kilovolts to be achieved. When a grounded rod (electrode) approaches the lifted metal plate, the high voltages cause a spark , an electrical discharge. Because of the spark width , it can easily be determined that the voltage to earth is considerably higher than when it was on the cake. The entire process can be repeated as often as you like, because the charges stored in the cake are not dissipated (“used up”) themselves. ${\displaystyle Q}$ ${\displaystyle C}$ ${\displaystyle Q}$ ${\displaystyle C}$ ${\displaystyle U}$

The electrical energy that is released during the brief spark discharge was previously applied against the force of the electrical field , in particular through the mechanical work when the metal plate was lifted off .

Applications of the principle

The principle of the electrophore can be implemented in an equivalent manner by charging the capacitor with an externally generated DC voltage instead of rubbing the dielectric of the capacitor before the plate is lifted off.

Variable capacitors in the form of a rotary capacitor are also suitable for the experiment : If this rotary capacitor is charged to its nominal voltage at maximum capacity, electrically isolated from the voltage source and then the capacitance is reduced by the rotary movement, the voltage between the plates increases to for electrical flashover between the plates.

The condenser microphone uses the sound-generated changes in the distance of a capacitor arrangement to generate a voltage change (LF signal). It requires pre-tensioning for charging. With the electret microphone , which works in a similar way, this charge is permanently stored in an electret film similar to the cake of the electrophore and does not have to be renewed.

The tape generator also uses the principle of the electrophore: the influential charge is generated either by static electricity (by lifting the insulating tape from the lower dielectric roll) or by spraying (tip discharge) onto the tape from an external voltage source. Charges on the belt are then moved away from the earth as the belt ascends. The Pelletron is a further development of the belt generator .

Practical use

The device is unlikely to have had any practical use, but was mainly used for basic research. The State Museum for Technology and Work in Mannheim called it "a very important device in the early days of research into electricity". It was further developed into an influence machine and thus enabled the later utilization of electrical energy.

Individual evidence

1. ↑ ^{a b} Elementa / Mannheim: Elektrophor ( Memento from October 15, 2007 in the Internet Archive )
2. ^ Silvanus Phillips Thompson: Elementary Lessons in Electricity & Magnetism . The MacMillan & Co, London 1891, p. 29-33 ( online ).

Web links

Commons : Electrophorus (device)  - collection of images, videos and audio files

• Description with illustration of the electrophore and the Bennet doubler derived from it