the blacking, and into which dips a copper cylinder having a grooved
surface. The horizontal axis of this cylinder is movable; when at rest it
is so placed that the cylinder is an inch or so below the brush, but when
the operator pulls a button that is within reach of his left hand, the
axis is lifted, a contact takes place between the brush and the cylinder,
and the former is thus given a rotary motion. As the cylinder still
continues to dip into the blacking, the latter is thus spread ever the
brush.--_La Genie Civil_.

* * * * *

PERSONAL SAFETY WITH THE ELECTRIC CURRENTS.

_To the Editor of the Scientific American_:

In your paper of the 21st of February there is an article on personal
safety with electric currents, by Prof. A.E. Dolbear. He says that a Holtz
machine may give through a short wire a very strong current. For if E =
50,000 volts, R = 0.001 ohm, then C = 50000/0.001 = 50,000,000 amperes.
Now that is a very large quantity of electricity, and is equal to an
enormous horse power. I think the person receiving that charge would not
need another. According to Ohm's law, the strength of current is
proportional to the electromotive force divided by the total resistance,
external and internal. The last is a very important element in the Holtz
machine, and will make a big difference in the current strength. Here are
some of the results obtained from experiments made with the Holtz machine.
A machine with a plate 46 in. in diameter, making 5 turns in 3 seconds,
produced a constant current capable of decomposing 31/2 millionths of a
milligram in a second. This is equal to the effect produced by a Grove's
cell in a circuit of 45,000 ohms resistance. The current produced would be
about 0.0000044 ampere. That is rather small compared with the Professor's
result. Rossetti found that the current is nearly proportional to the
velocity of rotation. It increases a little faster than the velocity.

The electromotive force and resistance is constant if the velocity is
constant. The electromotive force is independent of the velocity, but
diminishes as the moisture increases, and is about equal to 52,000 Daniell
cells. The resistance when making 120 revolutions per minute is 2,810
million ohms. At 450 per minute, 646 million.

Taking it at 450, C = 53950/64600000.001 = 0.0000835 ampere, against the
Professor's 50,000,000, amperes, and it would be equal to about 0.006
horse power, whi