582 REPORT— 1887. 



FRIDAY, SEPTEMBER 2. 

 The following Reports and Papers were read : — 



1, 



Fourth Report of the Committee for considering the test methods of re- 

 cording the direct Intensity of Solar Radiation. — See Reports, p. 32. 



2. Third Report of the Committee for considering the best means of com- 

 paring and reducing Magnetic Observations.— See Reports, p. 320. 



3. New Electric Balances.^ By Professor Sir William Thomson, F.R.S. 



The 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 horizontal, to the ends of the beam of a balance, and are each acted on by 

 two horizontal fixed rings placed one above and the 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 wiU measure currents of from 

 100 to 2,500 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 constant of which depends ou the strength of per- 

 manent magnets. 



The most important novelty in these balances is the connection between the 

 movable and the fixed part 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 

 ligaments serve instead of knife-edges for the balance, and at the same time allow 

 the current to pass into and out from the movable coils. The number of wires in. 

 each ligament varies I'rom 20 in the centi-ampere to 900 in the kUo-ampere 

 balance. The diameter of the wire is about j^th 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 num- 

 bers 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 accuracy for most purposes. A\'hen 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 in the manner above described, 

 and acted on by one fixed coil placed below it. The distance of the two coils apart 

 is indicated on a vertical scale by means of a magnifying lever, and serves to indicate 

 tlie diiference of potential between the leads to which the instrument is connected. 

 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 indi- 

 cated 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-light circuit on board ships at sea, and 

 a magneto-static current-meter suitable for a lamp-counter. 



' See Electrician, May 6, 13, and 20, 1887 ; also Tdcgrapliic Rericw. . 



