TUASSACTIONS OF SECTION A. 



t)3» 



m 



inaudible. "NVitli such a cablo tlie practical limit would uot bo likely to exccctl 

 fifty miles, more especially as the easy iiitellifjibility of speech requires the presence 

 of notes still higher than is supposed in the above numerical example. 



C.G.S. 

 ampli- 



re have 



almost 

 >ring it 



8. On the Influence of Marpietism on tlie Discharge of Electricity tlirmirjh 

 Gases. I'll Professor Aktiiuu Schuster, F.B.S. 



9. On. a Galvanometer with Tnynti/ Wires. 

 By Professor Lord Ravlkigii, LL.I)., F.B.S. 



Galvanometers suitable for currents of an ampere or two are most accnrati'ly 

 standardised by means of the silver voltameter, but this method ceases to ))o con- 

 venient when the current to be dealt with rises above live amperes. The present 

 instrument is a kind of diil'erential galvanometer, provided with two electrically 

 distinct coils, whose constants are in the ratio of ten to one. A current of one 

 ampere ihrouo-li one coil thus balances a current of ten amperes througli tlie other. 

 If the first be measured in terms of silver, the second serves to standardi.-e an 

 instrument suitable for the larger current. 



The novelty consists in the manner in which tlie ten to one ratio is secured.. 

 Twenty pieces of No. 17 cotton-covered wire, being cut to equal lengths of about 

 eight feet, were twisted closely together, two and two, so as to form ten pairs, which 

 ten pairs were again in their turn twisted sliglitly together so as to form a rope. 

 In each of the two circuits there are tim wires. In one, that intendeil for the 

 larger current, these -wives are in parallel ; in the other circuit the ten wires are iu 

 i5eries. Of each of the original twists one wire belongs to the parallel and one to 

 tlie series group. Now the two wires ibi'ining an original twist are equally effec- 

 tive upon a needle suspended in any reasonable situation with respect to them, 

 and thus if the ten wires in parallel have the same resistance, the circuit formed by 

 the ten wires in series will be precisely ten times as effective as the circuit formed 

 by the ten wires in parallel. This is independent of the disposition of tht) teii. 

 original pairs, but by winding tliem loosely into a rope we gain an additional 

 •security in case the ten parallel wires, though of the same length and cut from the 

 same hank, should have slightly different resistances. If all the twenty wires 

 could be assumed to have equal efhciency in deflecting the needle, the equality of 

 resistances of the wires in parallel would be of no moment. 



The rope is bent into a single circle of about a loot diameter with leads two 

 feet long. At this distance the necessary junctions can be effected without fear of 

 disturbance. The electrodes for tlie heavy current are formed of parallel 

 copper strips, separated by an insulating layer, and the current is brought up 

 through twisted leads as in Sir W. Thomson's graded galvanometers. In the case of 

 the smaller current, which (embraces the needle ten tinu's, so much precaution is not 

 required. 



After the wires in parallel had been soldered up, but while those destined to be 

 joined in series were still disconnected, insulation tests were made between each 

 wire of the series group and the other wires of that group as well as the group in 

 parallel. The resistance between each series wire and the parallel group was about 

 '2^ megohms, and (as might be expected) about twice as much between any pair of 

 wires of the series group. 



It will be feen that when, in tite use of the instruments, two currents are 

 balancing one another, every one of the twenty wires carries the same current. la 

 the actual instrument this current might amount, without undue heating, to four 

 amperes, so that the heavy current would be 40 amperes. If it be not thought 

 necessary to deal with currents heavier than 10 amperes, the gauge of wire might 

 be reduced, a change which would facilitate the winding of the rope. 



The magnet and mirror should l)e of the kind used in reflecting galvanometers, 

 and may be hung at the centre of the circle. 



V 1 



