TRANSACTIONS OF SECTION A. (J 33 



inaudible. With such a cable the practical limit would not be likely to exceed 

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

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



8. On the Influence of Magnetism on the Discharge of Electricity through 

 Gases. By Professor Arthur Schuster, F.R.S. 



9. On a Galvanometer with Twenty Wires. 

 By Professor Lord Eatleigh, LL.B., F.B.S. 



Galvanometers suitable for currents of an ampere or two are most accurately 

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

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

 instrument is a kind of differential galvanometer, provided with two electrically 

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

 ampere through one coil thus balances a current of ten amperes through the other. 

 If the first be measured hi terms of silver, the second serves to standardise an 

 instrument suitable for the larger current. 



The novelty consists in the manner in which the 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 slightly together so as to form a rope. 

 In each of the two circuits there are ten wires. In one, that intended for tbe 

 larger current, these wires are in parallel ; in the other circuit the ten wires are in. 

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

 the series group. Now the two wires forming 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 the ten 

 original pairs, but by winding them 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 efficiency 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 foot diameter with leads two 

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

 disturbance. The electrodes for the 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 times, so much precaution is not 

 required. 



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

 joined iu 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 seen that when, in the use of the instruments, two currents are 

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

 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 be of the kind used in reflecting galvanometers, 

 and may be hung at the centre of the circle. 



