Intelligence and Miscellaneous Articles. 517 
eylindrical shaft A (fig. 1), which is stationary. A belt passing 
over the pulley B turns the wheel ©, upon the face D of which 
Fig. 1. Fig. 2. 
revolve four brushes which connect the adjoining segments. The 
brushes aa, 66 are made adjustable, the two adjoining brushes 
being electrically connected, and a small stream of water plays 
upon the segments of the commutator. The character of the spark 
produced by a Ruhmkorff coil which is marked by alternating 
currents has been studied by Spottiswoode. Without condensers 
in the secondary circuit, a bright yellow glow spans the distance 
between the two terminals of the coil, which partakes more of the 
character of a voltaic arc than of the ordinary discharge from a 
Ruhmkorff coil. The apparatus which we used produced three 
thousand reversals a minute. This rate was too rapid for the best 
effects with a Ruhmkorff coil. It enabled us, however, to study 
the musical note produced in the cores of the electromagnet by 
rapid reversals of the current in the electromagnet, and also the 
heating effects which have been so often studied.—Silliman’s 
American Journal, May 1885. 
Jefferson Physical Laboratory. 
THERMO-ELECTRO-PHOTO-BARIC UNIT. 
BY PLINY EARI.E CHASE, LL.D. 
The earliest attempt at measurement, with a view to demonstrate 
the correlation of thermal and electric energies, appears to have 
been that of Principal Forbes, who found, in 1832 (Phil. Mag. iv. 
p- 27), that the conductivity of metals for heat and electricity is 
nearly the same. The dimensions of absolute measure involved 
were MV, L. 
In 1843 Joule published his discussion of the calorific effects 
of magneto-electricity, and his determination of the mechanical 
equivalent of heat (Phil. Mag. xxii. pp. 263, 347, 485), using the 
same dimensions, M, L. 
~ In 1856 Weber extended the correlation of Forbes (Pogg. Ann. 
xcix.), by showing the approximate equality of the electro-magnetie 
ratio to the velocity of light (£ 7'—'). | 
Wolf’s discovery of the sun-spot period was followed, in 1857, 
by the investigations of Lamont and Sabine, showing the identity 
