TRANSACTIONS OF SECTION A. 907 
10. On a method of multiplying Potential from a hundred to several thou- 
sand Volts. By Professor Sir Wittiam Toomson, LL.D., F.R.S. 
The method described by the author was to arrange in series a number n of 
condensers, where m is the number indicating the required multiplication, A 
terminal is connected to the junction between each pair of adjacent condensers. 
This series of n+1 terminals is conveniently placed so that by a suitable mechanism 
a pair of movable electrodes, between which a known difference of potentials exists, 
may be brought successively and repeatedly, at short intervals of time, into con- 
tact with each pair of adjacent terminals in the series, moving always in the same 
direction along them. In this way the difference of potentials established between 
the two end terminals of the series of condensers is m times the known difference 
of potentials between the movable electrodes. 
11, On a form of Mercury Contact Commutator of Constant Resistance for 
use in adjusting Resistance Coils by Wheatstone’s Bridge, and for other 
purposes. By Professor J. Viriamu Jones. 
The author was first led to consider commutators of the kind described by two 
needs: (1) In simple experiments on the induction of electric currents by the 
solution of a current in a magnetic field, it was desirable to substitute mercury 
contacts for the brass brass and copper copper contacts of the simple split ring 
commutator, and (2) for experiments on the resistance of electrolytes it is necessary 
to continue rapidly reversing the current without altering the resistance of the 
circuit. 
The simple commutator with mercury contacts consists of two co-axal, parallel, 
and equal split rings of copper, the lines of split being parallel to one another, and 
the halves being cross-connected. The split rings are supported on ebonite discs. 
If it is desired to convert a continuous current into an alternating current without 
change of resistance in the circuit, it is only necessary to add to the above simple 
commutator two other (unsplit) rings of copper properly connected and turning in 
mercury cups communicating with the terminals of the part of the circuit in which 
the current is continuous. 
If more rapid commutation is required, then by dividing the copper rings of the 
simple commutator into say six parts, and cross-connecting these, the current will 
be reversed six times in each revolution. 
By means of eight discs, each carrying a copper ring divided into two parts, the 
lines of split in the first, fourth, fifth, and eighth, being all parallel and at right 
angles to the lines of split of the second, third, fifth, and sixth, which are also 
all parallel, we may by proper connections change the direction of the current 
through two independent coils, then interchange the coils, and again reverse the 
direction of the current through the coils in their new position. Such a commu- 
tator, it will be perceived, is of great utility in adjusting resistance coils by 
Wheatstone’s bridge. 
12. On Slide Resistance Coils with Mercury Contacts. 
By Professor J. Viriamu Jones. 
13. On the relative Merits of Iron and Copper Wire for Telegraph Lines. 
By W.H. Preece, F.R.S. 
Copper is gradually replacing iron for aérial telegraphs, owing to its greater 
durability in the atmosphere ; but its greater cost has led to the use of smaller- 
sized wires. This can be done without detriment to the economy of the line, for 
the resistance of copper, as compared with iron, varies very nearly inversely as its 
price per ton, and hence the cost per mile remains about the same. Hitherto only 
short lengths have been erected in smoky towns and through districts where 
