PROFESSOR THOMSON ON THE ELECTRO-DYNAMIC QUALITIES OF METALS. 695 
of requiring no other apparatus than the commutator, in regular use in all applications 
of the battery, and it had been found to answer the purpose tolerably well in the first 
trial. It proved, however, very inconvenient with the finer conductors, from the too 
great abruptness of its action. Besides, it was open to this very serious objection, 
that it kept up the required heating effect by an intermittent current, and therefore 
by the passage of a much less quantity of electricity than would be required to 
produce the same heating effect if flowing in a nearly constant current (the rate of 
generation of heat being proportional to the square of the strength of the current at 
each instant, while the looked-for convective effect is proportional simply to the 
strength of the current at each instant, and is therefore, on the whole, proportional 
to the whole quantity of electricity that passes). 
In order, therefore, that the current might be kept as nearly as possible constant 
at the particular strength required to maintain the heating effect used, I had the 
following regulator constructed. 
71 . Two iron tubes, AB, CD, 20 inches long and fths of an inch in diameter, open 
at the top but closed at the bottom, are bound firmly together with insulating blocks 
of wood, AC and BD, so as to be parallel to one another. Pieces of thin sheet copper 
are bent into cylinders ; to their tops pieces of thick copper, E, E, are soldered, and 
the copper cylinders are put into the iron tubes. To each end of a piece of thick 
copper wire, shown separately at F, two pieces of No. 18 iron 
wire are fixed, one of the same length as the iron tubes, and 
the other less than half that length, and the two branches are 
parallel, and at such a distance that when their ends are in- 
troduced into the two tubes, they move along their axes. To use 
the regulator, the tubes are filled with mercury, the apparatus 
is put into the circuit by connecting with EE, and the requisite 
amount of resistance is introduced by raising G, which is kept 
in any position by having one end of a cord fixed to its upper 
part, carried over a pulley, and stretched by a counterpoise hung 
at its other end. [Great improvement has been since made in 
the regulator, by using, instead of No. 18 iron wire, thick copper 
wire tapering to points at the lower ends ; and by attaching 
cups of gutta percha to the tops of the iron tubes allowed to 
communicate with the interior by small holes, to serve as over- 
flow cisterns for the mercury. By this arrangement the tubes were kept always full 
of mercury, and irregular contacts between the connecting conductor and the interior 
of empty parts of the tubes were prevented. — Nov. 1856.] 
72 . The apparatus was set up as shown in the accompanying view. The battery 
connexions were completed with the regulating break partly up, so as to check the 
current somewhat, and prevent injury from sudden overheating in any part of the 
conductor. 
