434 PROCEEDINGS OF THE AMERICAN ACADEMY 



form many years ago in Nicholson's Mechanics and elsewhere. Three 

 vertical gas-pipes are placed side by side, and connected together below ; 

 then half filled with mercury, and so mounted that they may revolve 

 around the axis of the central pipe ; a glass tube filled with water is 

 attached to the latter, and serves to show the position of the mercury. 

 Motion was transmitted to the whole from the horizontal shaft of the 

 machine by a spiral spring, as in a dental lathe, but afterwards this 

 was replaced by a pair of bevel-geai's. If now the machine is set in 

 motion, the mercury is by centrifugal force thrown from the central to 

 the outer tubes, and the water in the glass tube falls. A graduated 

 scale shows the position of the water, wdaich remains very constant as 

 long as the velocity is uniform, and by its motion shows the slightest 

 variation in speed. The reduction is effected by noting the water level 

 with various velocities as measured by a shaft-speeder, and constructing 

 a curve with coordinates equal to these two quantities. If the tubes 

 are exactly parallel and of uniform diameter, this curve will be a 

 parabola, with axis vertical and parameter determined by the equation 

 3/= 473 X 10~^n^d', in which d is the distance of the outer tubes from 

 the centre in inches, and n the number of turns per second. Evidently 

 an inch would correspond to a much greater change in velocity at high 

 than at low speeds, and accordingly the open ends of the outer tubes 

 were bent in towards the centre. This had the additional advantage 

 of preventing the mercury from being thrown out, and of greatly in- 

 creasing the range of the instrument. As actually constructed, the 

 speed in turns per minute very nearly equalled the square of the 

 depression of the water level in tenths of an inch. 



Resistances. A difficulty at once j^resented itself in varying the 

 resistance of the circuit, since resistance coils of the ordinary form 

 would be at once injured or even melted by the immense quantity of 

 electricity transmitted. Accordingly a set of resistances were prepared 

 by stretching some uncovered German silver wire along the wall of 

 the laboratory, so as to form nine loops, of eighty feet each, of No. 28 

 wire. As the diameter is .017 inches, the surface exposed to radia- 

 tion is about 460 square inches; and, as the air circulates freely 

 around them, there is no diificulty from their heating, even when the 

 machine is connected directly with their terminals. Each of these 

 loops has a resistance of 3G.9 ohms, and one or more may be thrown 

 into circuit by a switch. For smaller resistances, a similar device was 

 employed. A frame, 3 feet wide and 6 feet high, was covered on both 

 sides with horizontal wires, passing around screws so as to form 30 

 loops of No. 22 wire (diameter .029") and 55 loops of No. 16 wire 



