MAGNETISM AND ELECTRICITY. 167 
in such a manner that the four movable metal rods, a’, a’, a’, a’ (fig. 37), cor- 
responding to the four rows of inlaid metal strips (fig. 38), fall down by their 
own weight, resting on these circles of strips, and thus conduct the current 
trom the fixed to the movable coils. Of the four rods, the two inner and 
the two outer ones communicate with each other alone, while the 
combination of the rows of strips in fig. 38 is just the reverse. When the 
axle with the commutator is set in motion it will be seen that two 
contiguous rods, a’, a’, alternately restore and interrupt the conduction to 
the commutator, and by the varying combinations in the two systems 
produce an alternation in the direction of the current in the movable coils. 
The battery employed with this machine is a Daniel’s battery improved by 
Stéhrer. Copper cylinders, v, v, v, v (fig. 36), have expansions above in which 
crystals of sulphate of copper are laid. Inside of the cylinder hangs a 
hempen bag with a wooden bottom ; on this is placed a cylinder of cast or 
sheet zine. The bag is filled with very dilute sulphuric acid, and the 
copper cylinder outside of the bag with sulphate of copper. The action of 
the acid on the zine results in the development of a current and the 
decomposition of part of the water combined with the acid. The hydrogen 
carried along with the positive current passes through the bag, and 
uniting with the oxyde of the sulphate of copper, liberates sulphuric acid 
and metallic copper. In combining the four batteries represented in the 
plate the usual method is employed, the zinc of the first and the copper of 
the last being left free for connexion with the wires communicating with 
the machine. This, when in full operation, may have a velocity amounting 
to 230 or 240 rotations in a minute. 
The apparatus represented in pl. 22, fig. 39, an improvement of 
Ritchie’s apparatus, is of similar construction. . Here AB is a large 
horse-shoe bar of soft iron, fastened to a wooden frame and wound with 
copper wire, whose extremities are conducted to the brass posts, a and 4, 
having binding screws above. If the electrodes of a strong galvanic 
battery be screwed to these posts, the iron AB will become converted into 
a magnet. Within this iron is a second, CD, of similar shape but of 
smaller size. This rotates on a vertical axis, and is also wound with 
copper wire, whose two extremities dip into a circular channel filled with 
mercury. The channel is separated into two semicircles by bridges of 
wood or cork, each of which is connected conductively with one of the 
posts, c and d. ‘The channel is so filled with mercury that this projects 
slightly above the bridges without running over, owing to its capillarity. 
The wires dip into the mercury enough to insure conducting communication 
and yet not enough to touch the bridges. If the positive electrode of a 
battery be screwed in c, and the negative in d, then in the position repre- 
sented in the figure, the positive current will pass from c¢ into the left 
division of the channel, then round through the coil of copper wire from 
D to C, thence through the right division of the channel to d. In this 
instance the pole C is attracted by A, and D by B, thereby producing a 
partial rotation of the electro-magnet, CD. When C has reached A, and 
I) has reached B, the extremities of the rotating wires cross the bridges 
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