HISTORY.] ELECTRICITY 13 leasure- ocnt of urrent. Jalvano- nsters. ilectro- uotive orce and nternal esist- mce. Lave been made in our time. One of the first to use au electro-magnetic instrument for measuring or indicating currents was Schweigger, who in 1820 invented the " multiplier." Nobili used (1825) the astatic " multiplier" with two needles, which is sometimes named after him. Becquerel (1837) used the electromagnetic balance, which was employed in an improved form by Lenz and Jacobi. Pouillet invented the sine and tangent compasses (1837). The defects of the latter instrument were pointed out by Poggendorff, and remedies suggested by him as well as Wheatstone and others. Weber effected great improve ments in the construction and use of galvanometers, adapted them for the measurement of transient currents, and elaborated the method of oscillations which had been much used by Fechuer. In 1849 Helmholtz invented the tangent compass with two coils which bears his name. Great improvements in delicacy and promptness of action have been made by Sir William Thomson in galvanometers destined for the measurement of resistance, and for indi cating the feeble currents of submarine cables. The measurement of resistance has been carried to great perfection, chiefly owing to the labours of those who have busied themselves in perfecting the electric telegraph. Among such the highest place must be assigned to Sir Charles Wheatstone; his memoirs in the Philosophical Transactions (1843) gave a great impulse to this depart ment of our science. He invented the rheostat, which underwent several modifications, but is now superseded by the resistance box which was first used by Siemens. The earlier methods of Ohm, Wheatstone, and others for measuring resistance were defective, because they de pended on the constancy of the battery which furnished the current. These defects are completely obviated in the more modern " null methods," which may be divided into two classes those which depend on the use of the differential galvanometer introduced by Becquerel, and those which are modifications of- the Wheatstone s bridge method, invented by Christie and brought into use by Wheatstone. As examples of the latter, we may mention the methods of Thomson, and of Matthiessen and Hockin, for measuring small resistances, and Thomson s method for measuring the resistance of the galvanometer (see Max well s Electricity and Magnetism, pp. 404, 410) Many determinations of the specific resistances of metals and alloys have been made by Davy, Ohm, Becquerel, Matthies sen, and others. To Matthiessen in particular science is indebted for great improvements in method and a large body of valuable results in this department. The metals have been arranged in a series according to their conducting powers ; and this series is found to be nearly the same for electricity as for heat. The conductivity of metals decreases as the temperature in creases, the rate of decrease being nearly the same for most pure metals, but much smaller and more variable for alloys, which, on the other hand, have in general a large specific resistance. The earlier attempts to measure the resistance of electrolytes were not satisfactory, owing to insufficient allowance for polarization. In later times this difficulty has been overcome or avoided, and concordant results have been obtained by Beetz, Paalzow, Kohlrausch, Nippoldt, and Grotrian. The three last, using the electro- dynamometer and sine inductor, have made elaborate re searches, establishing among many other interesting re sults that the conductivity of electrolytes increases with the temperature (Pogg. Ann., 1869-74). The measurement of the electromotive force and that of internal resistance of batteries in action are problems which, in their most general form, are inextricably connected. It is easy to measure with considerable accuracy the electro motive force of an open battery. We have merely to connect its poles with a Thomson s electrometer, and compare the deflection thus obtained with that due to some standard electromotive force. Another very satis factory method is Latimer Clarke s modification of Poggen- dorff s compensation method (see Maxwell, 413). It is likewise not difficult to measure by a variety of methods, the most satisfactory being that of Mance (Maxwell, 411), the internal resistance of a battery when it is only traversed by a feeble current. But the measurement of the electro motive force and internal resistance of a battery working a strong current has hardly as yet been achieved with success ; not that we undervalue the ingenious and important methods of Paalzow, Von Waltenhofen, Beetz (Wiede- mann, i. 181), and Siemens (Pogg. Ann., 1874). The concordant results of the last two are indeed very remarkable. Still all these methods are more or less affected by the fact that the electromotive force of a battery depends on the current which it is sending (see Beetz in Pogg. Ann., cxlii.). The " crown of cups " of Volta was the parent of a Batter great many other arrangements for the production of voltaic electricity. These had for their end either com pactness or diminution of the internal resistance by en larging the plates ; we may mention the batteries of Cruickshank (1801), Wollaston (1815), and Hare (1822). In 1830 Sturgeon introduced the capital improvement of amalgamating the zinc plates. In 1840 Smee used platinum or silver plates instead of copper ; by platinizing these he avoided to a considerable extent polarization by adhering hydrogen. In 1836 Daniell invented the two- fluid battery which bears his name. This battery is the best constant battery hitherto invented, and is, under various modifications, largely used in practical and scien tific work. In the same year Grove invented his well- known battery, which surpasses Daniell s in smallness of internal resistance and in electromotive force, although, on the other hand, it is more troublesome to manage and is unsuited for long-continued action. Cooper, in 1840, replaced the expensive platinum plates of Grove s battery by carbon. This modification was introduced in a prac tical form into the battery of Bunsen (1842), which is much used on the Continent, and combines to a certain extent the advantages of Grove and Daniell. Among the more recent of one-fluid batteries may be mentioned the bichromate battery of Bunsen and the Leclanche" cell. It is impossible here even to allude to all the forms of battery that have been invented. We may, however, in passing notice the gravitation batteries of Meidinger and Varley, and the large tray cell of Sir William Thomson. Following up the discoveries of Nicholson, Carlisle, Electr Davy, and others, Faraday took up the investigation of 1 > rsis - the chemical decompositions effected by the electric current. In 1833 he announced his great law of electro-chemical equivalents, which made an epoch in the history of this part of electricity. He recognized and for the first time thoroughly explained the secondary actions which had hitherto masked the essential features of the phenomenon. Faraday s discovery gave a new measure of the current, and he invented an instrument called the voltameter, which was much used by those who followed out his discoveries. Space fails us to notice in detail the labours of those who verified and developed Faraday s discovery. De la Rive, Becquerel, Soret, Buff, Beetz, Hittorf, Mat- teucci, Daniell, Miller, and many others have worked in this field. Many theories of electrolysis have been given. That of Theori Grotthuss (1805) has been held under various modifi- of ^ ec cations by many physicists ; but none of these theories " have done more than give us a convenient mode of repre
senting experimental results. Clausius (Pogg. Ann.,