522 



NATURE 



[Sept. 29, 1887 



This he gives as !h3 complete expression ; wherein, therefore, 



T 

 J0€ is the chemical poriion of the total E.M.F., and ]de — - 



^ 

 the thermal portion of the whole E.M. F., equal to J2n. 



If this were a correct mode of regarding the matter, it would 

 be of the highest interest to be able to calculate dissociation 

 temperatures in this way. Unfortunately, several of the best 

 judges in this country have expressed to the Committee their 

 serious doubts as to the validity of thus stepping, unguided, out- 

 side the region of safe knowledge, across the great gap separating 

 ordinary from dissociation temperatures. We wish Prof Willard 

 Gibbs were here to support and strengthen his position. 



These are the main problems at present under discussion 

 among the members of the Committee, and with this summary of 

 them and reference to such of to-day's papers as seem likely to 

 contribute towards their solution the report proper may be under- 

 stood to close. 



I think, however, I am only expressing the feeling of the Com- 

 mittee if I say that they view this joint sitting of Sections A and 

 B with great interest, and with the anticipation and hope that it 

 may be the precursor of many other such gatherings during the 

 era of development in the borderland of chemistry and physics 

 which in many directions they feel to be now imminent. 



Section A — Mathematical and Physical Science. 



Ne7v Electric Balances, by Sir "William Thomson, F.R.S. — 

 These balances are founded on the mutual forces,- discovered by 

 Ampere, between the fixed and movable portions of an electric 

 circuit. The mutually-influencing portions are usually circular 

 rings. Circular coils or rings are fixed, with their planes hori- 

 zontal, to the ends of the beam of a balance, and are each acted 

 on by two horizontal fixed rings placed one above and tlie other 

 below the movable ring. Six grades of instrument are made, 

 named centi-ampere, deci-ampere, ampere, deca-ampere, hecto- 

 ampere, and kilo-ampere balance. The range of each balance 

 is about 25. Thus, the centi-ampere balance will measure 

 currents of from 2 to 50 centi-amperes, while the kilo-ampere 

 balance will measure currents of from 100 to 2500 amperes. 

 Since the indications of the instrument depend on the mutual 

 forces between two parts of an electric circuit of permanent form 

 and relative position, they are not subject to the changes with 

 time which are so troublesome in instruments the constants of 

 which depend on the strength of permanent magnets. 



The most important novelty in these balances is the con- 

 nexion between the movable and the fixed parts of the circuit. 

 The beam of the balance is suspended by two flat ligaments 

 made up of fine copper wires placed side by side. These liga- 

 ments serve instead of knife-edges for the balance, and at the 

 same time allow the current to pass into and out from the mov- 

 able coils. The number of wires in each ligament varies from 

 20 in the centi-ampere to 900 in the kilo-ampere balance. The 

 diameter of the wire is about jV of a millimetre, and each centi- 

 metre breadth of the ligament contains about 100 wires. 



The electric forces produced by the current are balanced by, 

 means of weights which can be moved along a graduated scale 

 by means of a self-relieving pendant. Two scales are provided — 

 one a scale of equal divisions, the other a scale the numbers on 

 which are double the square roots of the numbers on the scale of 

 equal divisions. The square-root scale allows the current to be 

 read off directly to a sufficient degree of accu-acy for most pur- 

 poses. When high accuracy is required, the fine scale of equal 

 divisions may be used, and the exact value of the current 

 obtained from a table of doubled square roots supplied with the 

 instrument. 



An engine-room voltmeter on a similar plan was described. 

 It consists of a coil fixed to the end of a balance arm (suspended 

 as above described) and acted on by one fixed coil placed below 

 it. The distance apart of the two coils is indicated by mear^s 

 of a magnifying lever, and serves to indicate the difference of 

 potential between the leads to which the instrument is con- 

 nected. The coils of the instrument are of copper wire, and an 

 external platinoid resistance of considerably greater amount is 

 joined in circuit with it. The electrical forces are balanced by 

 means of a weight placed in a trough fixed to the front of the 

 movable coil and weights suited to the temperatures 15°, 20°, 

 25°> 30° C., as indicated by a thermometer with its bulb in the 

 centre of the coil, are provided. 



Two other instruments were described — namely, a marine 



voltmeter suitable for measuring the potential of an electric 

 circuit on board ship at sea, and a magneto-static current-meter 

 suitable for a lamp-counter. 



In the marine voltmeter an oblate spheroid of foft iron is 

 suspended in the centre of, and with its equatorial plane inclined 

 at about 40° to, the axis of a small coil of fine wire, by means 

 of a stretched platinoid wire. Wtien a current is passed 

 through the coil, the oblate of soft iron tends to set its equa- 

 torial plane parallel to the axis of the coil, and this tendency is 

 resisted by the rigidity of the suspension wire. 



The lamp-counter is a tangent galvanometer with special 

 provision for preventing damage to its silk fibre suspension, and 

 for allowing the constant to be readily varied by the user to suit 

 the lamps on his circuit. 



O71 the Application of the Centi-ampere or the Deci-ampere 

 Balance for the Measurement of the E. M. F. of a Single Cell, 

 by Sir William Thomson, F. R. S. — For the purpose of measuring 

 the E.M.F. of a single cell, the centi-ampere or the deci-ampere 

 balance is put in circuit with a battery of a sufficient number of 

 cells, a rheostat, and a standard resistance in the manner shown 

 in the diagram. The current measured by the balai.ce is then 



varied by means of the rheostat untilUhe difterence of potential 

 between the ends of the standard resistance is exactly equal to 

 the potential of the cell. The equality is tested by placing the 

 cell in series with a mirror galvanometer or a quadrant electro- 

 meter in a derived circuit, the ends of which are connected with 

 the ends of the standard resistance, and observing whether aT>y 

 deflection is obtained by closing this circuit. Suppose, for 

 example, the standard resistance to be 10 ohms, and the current, 

 as indicated by the balance, o"io8 amperes, when no deflection 

 is obtained on the mirror galvanometer by closing its circuit, the 

 potential of the cell is 10 x o'loS, or i"o8 volts. Proper pre- 

 cautions must, of course, be taken to eliminate thermo-electric 

 or other disturbances in the circuit. The quadrant electrometer 

 may be used with advantage in the derived circuit when it is 

 important that no current should flow through the cell, but the 

 mirror galvanometer has the advantage of much greater sensi- 

 bility. 



Conduction of Electricity through Gases, by Prof. A, 

 Schuster, F.R.S. — A short time ago he communicated to the 

 Royal Society the results of certain experiments which showed 

 that, although a current can usually be sent through a gas only 

 by employing a high electromotive force, yet a steady current 

 can be obtained in air from electrodes which are at a diff"erence 

 of potential of only a fractijn of a volt, provided that an inde* 



