The invention of the portable electrometer and the water-dropping electrograph by Lord Kelvin in the middle of the 19th century, and the greater definiteness thus introduced into observational results, were notable events.
Of flame collectors the two best known are Lord Kelvin's portable electrometer with a fuse, or F.
The height of the walls in the various observatories, the height of the collectors, and the distance they project from the wall vary largely, and sometimes electrometer, and they sometimes leave hardly a trace on the photographic paper.
On the other hand, it is commonly thought that the single potentialdifferences at the surface of metals and electrolytes have been determined by methods based on the use of the capillary electrometer and on others depending on what is called a dropping electrode, that is, mercury dropping rapidly into an electrolyte and forming a cell with the mercury at rest in the bottom of the vessel.
De Phys., Paris, 1900, p. 561) that the true effect of magnetization is liable to be disguised by secondary or parasitic phenomena, arising chiefly from polarization of the electrodes and from local variations in the concentration and magnetic condition of the electrolyte; these may be avoided by working with weak solutions, exposing only a small surface in a non-polar region of the metal, and substituting a capillary electrometer for the galvanometer generally used.
For this purpose the following apparatus should be provided: - (i) two small metal tea-trays and some clean dry tumblers, the latter preferably varnished with shellac varnish made with alcohol free from water; (2) two sheets of ebonite rather larger than the tea-trays; (3) a rod of sealing-wax or ebonite and a glass tube, also some pieces of silk and flannel; (4) a few small gilt pith balls suspended by dry silk threads; (5) a gold-leaf electroscope, and, if possible, a simple form of quadrant electrometer (see Electroscope and Electrometer); (6) some brass balls mounted on the ends of ebonite penholders, and a few tin canisters.
The instrument consists of a high-voltage continuous-current dynamo which creates a potential difference between the needle and the two quadrants of a quadrant electrometer (see Electrometer).
If the quadrants of an electrometer are con - nected to the ends of a non-inductive circuit in series with the power-absorbing circuit, and if the needle is connected to the end of this last circuit opposite to that at which the inductionless re - sistance is connected, then the deflexion of the electrometer will be proportional to the power taken up in the circuit, since it is pro - portional to the mean value of (A - B) IC - 1 (A ±B)}, where A and B are the potentials of the quadrants and C is that of the needle.
Ayrton, "Electrometer Methods of Measuring Alternating Current Power," Journ.
A voltmeter is therefore one form of electrometer, but the term is generally employed to describe the instrument which indicates on a scale, not merely in arbitrary units but directly in volts, the potential difference of its terminals.
The principles of telegraphy (land, submarine and wireless) and of telephony are discussed in the articles Telegraph and Telephone, and various electrical instruments are treated in separate articles such as Amperemeter; Electrometer; Galvanometer; Voltmeter; Wheatstone'S Bridge; Potentiometer; Meter, Electric; Electrophorus; Leyden Jar; &C.
The accuracy of his measurement, by which he established within 2% the above law, was only limited by the sensibility, or rather insensibility, of the pith ball electrometer, which was his only means of detecting the electric charge.2 In the accuracy of his quantitative measurements and the range of his researches and his combination of mathematical and physical knowledge, Cavendish may not inaptly be described as the Kelvin of the 18th century.
He ascertained the distribution of electricity among several spheres (whether equal or unequal) placed in contact in a straight line; and he measured the distribution of 2 In 1878 Clerk Maxwell repeated Cavendish's experiments with improved apparatus and the employment of a Kelvin quadrant electrometer as a means of detecting the absence of charge on the inner conductor after it had been connected to the outer case, and was thus able to show that if the law of electric attraction varies inversely as the nth power of the distance, then the exponent n must have a value of 2 t Isua.
Thomson), by the use of his then newly-invented electrometer, was able to confirm Volta's observations on contact electricity by irrefutable evidence, but the contact theory of the voltaic pile was then placed on a basis consistent with the principle of the conservation of energy.
His ampere-balances, voltmeters and electrometers, and double bridge, are elsewhere described in detail (see Amperemeter; Electrometer, and Wheatstone'S Bridge).
ELECTROMETER, an instrument for measuring difference of potential, which operates by means of electrostatic force and gives the measurement either in arbitrary or in absolute units (see Physical Units).
The simplest form of repulsion electrometer is W.
Henley's pith ball electrometer (Phil.
A double pith ball repulsion electrometer was employed by T.
It may be pointed out that such an arrangement is not merely an arbitrary electrometer, but may become an absolute electrometer within certain rough limits.
A form of attracted disk absolute electrometer was devised by A.
Snow-Harris found that this charge varied as the square root of the weight in the opposite pan, thus showing that the 1 It is probable that an experiment of this kind had been made as far back as 1746 by Daniel Gralath, of Danzig, who has some claims to have suggested the word " electrometer " in connexion with it.
His portable electrometer is shown in fig.
Lord Kelvin's absolute electrometer (fig.
Symmetrical Electrometers include the dry pile electrometer and Kelvin's quadrant electrometer.
In the dry pile electrometer a single gold-leaf is hung up between two plates which are connected to the opposite terminals of a dry pile so that a certain constant difference of potential exists between these plates.
Hankel still further improved the dry pile electrometer by giving a slow motion movement to the two plates, and substituted a galvanic battery with a large number of cells for the dry pile, and also employed a divided scale to measure the movements of the gold-leaf (Pogg.
A vast improvement in this instrument was made by the invention of the quadrant electrometer by Lord Kelvin, which is the most sensitive form of electrometer yet devised.
A replenisher or rotating electrophorus, by means of which the charge of the Leyden jar which forms the enclosing vessel can be increased or diminished, and also a small aluminium balance plate or gauge, which is in principle the same as the attracted disk portable electrometer by means of which the potential of the inner coating of the Leyden jar is preserved at a known value.
They found from observations that the particular quadrant electrometer they used might be made to follow one or other of three distinct laws.
In other words, the electrometer nearly obeyed the theoretical law.
Guided by these experiments, Ayrton, Perry and Sumpner constructed an improved unifilar quadrant electrometer which was not only more sensitive than the White pattern, but fulfilled the theoretical law of working.
The conclusion to which the above observers came was that any quadrant electrometer made in any manner does not See Maxwell, Electricity and Magnetism (2nd ed., Oxford, 1881), vol.
Necessarily obey a law of deflection making the deflections proportional to the potential difference of the quadrants, but that an electrometer can be constructed which does fulfil the above law.
The importance of this investigation resides in the fact that an electrometer of the above pattern can be used as a wattmeter, provided that the deflection of the needle is proportional to the potential difference of the quadrants.
Let v l, v 2, v 3 be the instantaneous potentials of the two ends and middle of the circuit; let a quadrant electrometer be connected first with the quadrants to the two ends of the inductive circuit and the needle to the far end of the non-inductive circuit, and then secondly with the needle connected to one of the quadrants (see fig.
Assuming the electrometer to obey the above-mentioned theoretical law, the first reading is proportional to v 1 -v 2) and the second to v, - v 2 The difference of the readings is then proportional to (0, - v2) (v, - v3).
But this last expression is proportional to the instantaneous power taken up in the inductive circuit, and hence the difference of the two readings of the electrometer is proportional to the mean power taken up in the circuit (Phil.
Blondlot and P. Curie afterwards suggested that a single electrometer could be constructed with two pairs of quadrants and a duplicate needle on one stem, so as to make two readings simultaneously and produce a deflection proportional at once to the power being taken up in the inductive circuit.
In this last case, however, the capacity of the electrometer used must be small, otherwise an error is introduced.'
He has constructed a capillary electrometer by which differences of electric potential less than o oi of that of a Daniell's cell can be detected by the difference of the pressure required to force the mercury to a given point of a fine capillary tube.