Voltameter
A voltameter is a historical measuring device used to measure electrical charge (amount of electricity). An alternative name is coulometer or coulomb meter . It was used in the form of the Edison counter to measure the amount of electricity used in the first electricity networks. Except in the field of training and for demonstration purposes, it no longer has any practical significance. It is not to be confused with a voltage measuring device , colloquially known as a voltmeter .
The term originated from a designation by Michael Faraday , who called this measuring device a "volta-electrometer". Subsequently, John Frederic Daniell called the structure "Voltameter".
In principle, the voltameter represents an ammeter that adds up over time . The SI unit for the electrical charge is the coulomb .
species
A voltameter is an electrolysis cell in which the electrical current causes a chemical conversion, e.g. B. gas evolution or metal deposition. The actual measurement is then carried out depending on the type of coulometer
- Weighing the chemical element that is deposited or released on one of the electrodes in a given time, or
- by measuring the volume of the gas or
- by titration.
Metal voltameter
A metal is electrically dissolved and redeposited, and the charge can be determined by weighing the dried electrodes. Most of the time, the metal deposited on the negative electrode, the cathode, was weighed. Since the electrode was usually removed and dried for weighing, these devices worked discontinuously.
Silver voltameter
The silver coulometer is the most accurate type. It consists of two silver plates in a solution of silver nitrate . When current flows, silver dissolves on the anode and deposits on the cathode. The cathode is weighed, the current flows in a measured time, then the cathode is weighed again, etc. The silver coulometer is of particular historical importance because it served as the official primary standard for measuring current strength until 1948 .
Copper voltameter
The copper coulometer is similar to the silver voltameter, but the anode and cathode are made of copper and the solution is copper sulfate dissolved in sulfuric acid . It is cheaper than the silver voltmeter, but a little less accurate.
Zinc voltameter
Zinc voltameters went down in the history of electricity as the first electricity measuring devices installed at customers: Thomas Alva Edison used them to measure electricity from 1881, and from 1883 had the customers' electricity bills for his direct current network drawn up with them. To improve accuracy and for control purposes, two zinc cells were connected in series; these in turn were not connected in the main circuit, but in parallel to a measuring resistor ( shunt ) in the main circuit.
Mercury voltameter
Volume measurement is also possible with the mercury coulometer. A particular embodiment utilizes a sliding drop of mercury, which in the first hour meter , such as the Edison meter was used to measure the amount of electricity-related.
Gas coulometer
Water was electrolyzed in these, usually using dilute sulfuric acid as the electrolyte. This is why these coulometers are also called sulfuric acid voltameters. The anode and cathode are preferably made of the corrosion-resistant platinum . During electrolysis, hydrogen is produced at the cathode and oxygen at the anode. The two gases were collected in the oxyhydrogen voltameter as a mixture in a measuring glass. The two gases are collected separately in the Hofmann voltameter or Hofmann water decomposition apparatus .
Since the gas volume depends on the temperature, more accurate devices were equipped with a thermometer, and the volume was converted to standard conditions . Since volume measurements are less accurate than mass determinations with a precision balance, gas coulometers were also less precise than metal coulometers. The great advantage of volume measurement, however, is that no electrode has to be removed, so that a quasi-continuous measurement is possible.
Other designs
Another design is the iodine coulometer , in which the amount of iodine produced at the anode was determined by titrating the solution.
Charge calculation using the electrochemical equivalent
According to Faraday's laws , the charge Q is proportional to the converted masses , and the following applies:
- Q : transferred electric charge to be determined
- Δ m : Change in mass of the anode or cathode in the case of metal electrodes, determined with the most accurate balance possible, or the amount of gas in the case of gas coulometers, which is calculated from the gas volume and the gas density
- z : number of electrons transferred per particle, valence of the element. For Ag / Ag + is z = 1 for Cu / Cu 2+ and Zn / Zn 2+ is z = 2
- F : Faraday constant (≈ 96,485.3 As mol −1 )
- M : molar mass . For silver: M = 107.8682 g / mol, for copper: M = 63.546 g / mol.
- Ä e : electrochemical equivalent , Ä e = M / z F . The electrochemical equivalent of an element is the mass of that element (in grams or milligrams ) carried by 1 coulomb of electricity.
| element | Atomic number | Molar mass in g / mol | Value | Ä e in mg / As | Ä e in g / Ah |
|---|---|---|---|---|---|
| copper | 29 | 63,546 | 2 | 0.3293 | 1.1855 |
| oxygen | 8th | 15.9994 | 2 | 0.0829 | 0.2985 |
| silver | 47 | 107.8682 | 1 | 1.1180 | 4.0247 |
| hydrogen | 1 | 1.00794 | 1 | 0.0104 | 0.0376 |
| zinc | 30th | 65,409 | 2 | 0.3390 | 1.2202 |
Other charge measuring devices
The measurement of the electrical charge can be done with less technical effort by electronic circuits such as the charge amplifier . This supplies an electrical voltage proportional to the amount of electrical charge.
literature
- WE Ayrton and T. Mather: Practical Electricity . Cassell and Company, London 1911, p. 12-26 .
- John Ellis Myers: About the silver voltameter and about Faraday's law for currents of frictional electricity . Buchdruckerei C. Gœller, Strasbourg i. E. 1895 ( archive.org ).
Web links
swell
- ^ Frank AJL James: The correspondence of Michael Faraday , 1991, IET, ISBN 0863412491 , letter 872, 9/1/1836