5o6 



NA TURE 



{Sept. 21, ib 82 



comet's visibility, he concluded that at the passage 

 through perihelion in October, 1844, the comet was 

 moving in an elliptical orbit with a period of revolution of 

 102050 ± 3090 years. In 1S46 he made extensive calcu- 

 lations bearing upon the motion of the two heads of 

 Biela's comet, the results of which will be found in No. 

 584 of the Astronomische Nachrichten. He further dis- 

 cussed the elements of what was called at the time 

 " Galle's second comet," 1840 II. (Astron. Nach., No. 

 475-6). In this paper he pointed out some anomalies in 

 the intensity of the comet's light, similar to what has 

 been observed from time to time in other comets. 



Plantamour was placed on the list of Associates of the 

 Royal Astronomical Society in 1844; he was a corre- 

 sponding member of the Academy of Sciences of the 

 Institute of France, and honorary member of the Academy 

 of Turin. Few of those colleagues who were at work at 

 the commencement of his astronomical life now remain. 



ON SIR WILLIAM THOMSON'S GRADED 

 GA L VA NOME TERS 



'TWENTY years ago the experimental sciences of 

 *■ electricity and magnetism were in great measure 

 mere collections of qualitative results, and, in a less 

 degree, of results quantatively estimated by means of 

 units which were altogether arbitrary. These units, de- 

 pending as they did on constants of instruments and 

 conditions of experimenting which could never be made 

 fully known to the scientific public, were a source of 

 much perplexity and labour to every investigator, and 

 to a great extent prevented the results which they 

 expressed from bearing fruit to the furtherance of 

 scientific progress. Now happily all this has been 

 changed. Ths absolute system of units introduced by 

 Gauss and Weber and rendered a practical reality in this 

 country by the labours of the British Association Committee 

 on Electrical Standards has changed experimental elec- 

 tricity and magnetism into sciences of which the very 

 essence is the most delicate and exact measurement, and 

 enables their results to be expressed in units which are 

 altogether independent of the instruments, the surround- 

 ings, and the locality of the investigator. 



The record of the determinations of units made by 

 members of the Committee, for the mcst part by methods 

 and instruments which they themselves invented, forms 

 one of the most interesting and instructive books in the 

 literature of electricity, and when the history of electrical 

 discovery is written the story of their work will form one 

 of its most important chapters. But besides placing 

 oa a sure foundation the system of absolute units, 

 they conferred a hardly less important benefit on elec- 

 tricians by giving them a convenient nomenclature for 

 electrical quantities. The great utility of the practical 

 units and nomenclature, which the Committee recom- 

 mended, soon became manifest to every one who had 

 to perform electrical measurements, and has led within 

 the last year to their adoption, with only slight al- 

 terations, by nearly all civilised nations. Although it 

 is not yet quite twelve months since the late Con- 

 gress of Electricians at Paris concluded its sittings, 

 the recommendations which it issued have been widely 

 adopted and appreciated by those engaged in electrical 

 work, and have thus begun to yield excellent fruit by 

 rendering immediately available for comparison and as a 

 basis for further re-:earch the results of experimenters in 

 all parts of the world. Soon even the ordinary workmen 

 in charge of dynamo machines or employed in electrical 

 laboratories will be able to tell the number of volts and 

 amperes which a generator can give at a certain speed 

 and under certain conditions, to determine the number of 

 amperes of current required to light an incandescence 

 lamp to its full brilliancy, or to measure the capacity of a 

 secondary cell in coulombs per square centimetre. 



But in order that the full benefit of the conclusions 

 of the Paris Congress may be obtained it is essential in 

 the first place that convenient instruments should be 

 used, adapted to give directly, or by an easy reduction 

 from their indications the number of amperes of current 

 flowing in a particular circuit, and the number of volts of 

 difference of potentials between any two points in that 

 circuit. To be generally useful in practice these instruments 

 should be easily portable, and should have a very large 

 range of sensibility ; so that, for example, the instrument, 

 which suffices to measure the full potential produced by 

 a large Siemens or Edison machine, may be also avail- 

 able for testing, if need be, the resistances of the various 

 parts of the armature and magnets by the only satisfac- 

 tory method ; namely by comparing by means of a 

 galvanometer of high resistance the difference of potential 

 between the two ends of the unknown resistance with 

 that between the ends of a known resistance joined up 

 in the same electrical circuit. In like manner the 

 ampere measurer should be one that could be introduced 

 without sensible disturbance into a circuit of low resis- 

 tance to measure either a small fraction of an ampere, or 

 the whole current flowing through a circuit containing a 

 large number of electric lamps. These conditions are 

 fulfilled by two instruments recently invented and patented 

 by Sir William Thomson and called by him Graded 

 Galvanometers. To give a short account of these instru- 

 ments is the object of the present article. 



/. The Potential Galvanometer . 



The galvanometer used for measuring differences of 

 potential in electrical circuits is shown in Fig. 1 which is 

 engraved from a photograph of the actual instrument. 

 It consists of two essential parts, a coil and a magneto- 

 meter. The coil is made of silk covered copper or 

 German silver wire of No. 32 B.W.G. When made 

 of German silver wire it contains about 2,200 yards of 

 wire wound in 7,000 turns, and has a resistance of over 

 6,000 ohms. It is made in the form of an anchor ring 

 having an outside diameter of fourteen centimetres 

 and an inside diameter of six centimetres. The 

 diameter of section is thus four centimetres. The coil 

 is wound within a mould of proper shape and dimensions, 

 and is then impregnated with melted paraffin under the 

 receiver of an air-pump. A solid compact ring is thus 

 obtained, which does not require a wooden case ; and 

 which served round with a covering of silk ribbon looks well 

 and is not at all liable to get out of order. The coil thus 

 constructed is attached to one end of the horizontal 

 wooden platform P shown in the drawing, and kept firmly 

 in its place by a pair of wooden clamps fitted to .the 

 lower half of the coil, and screwed firmly to the end of 

 the platform. When in position the plane of the coil is 

 vertical, and at right angles to a V groove that runs along 

 the middle of the platform. The centre of the coil is 

 opposite to and about one and a half centimetres above 

 the bottom of this groove. 



On the platform P rests the magnetometer M (shown 

 in plan in Fig. 2), which consists essentially of a sys- 

 tem of magnets properly supported so as to be free 

 to turn round a vertical axis, and shielded from cur- 

 rents of air by being enclosed in a quadrantal shaped 

 box having a closely fitted glass cover. Each magnet 

 is fully one centimetre in length, and is made of glass- 

 hard steel wire of No. 18 B.W.G. Four of these mag- 

 nets mounted in a frame with their poles turned in 

 similar directions from the "needle" of the instru- 

 ment. The frame carrying the magnets is made of two 

 thin bars of aluminium placed side by side with their 

 planes vertical and about a centimetre apart ; and con- 

 nected by a bridge of sheet aluminium. The ends of 

 the magnets are fixed in holes in the vertical sides of the 

 aluminium frame so that the four steel needles form a set 

 of four horizontal parallel edges of a rectangular prism. 



