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 3½ millionths of a
milligram in a second. This is equal to the effect produced by a Grove's