_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 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, which I think would be the more correct of the two; calling E equal to 50,000 Daniell cells. Yours, Respectfully, E. ELLSWORTH. Portland, Me., March 5, 1885. * * * * * A VISIT TO CANADA AND THE UNITED STATES IN THE YEAR 1884. [Footnote: A lecture delivered before the Society of Telegraph Engineers and Electricians, London, Dec. 11, 1884.] By Mr. W.H. PREECE, F.R.S. I do not know what the sensations of a man can be who is about to undergo the painful operation of execution; but I am inclined to think his sensations must be somewhat similar to those of a lecturer, brimful of notes, who has to wait until the clock strikes before he is allowed to address his audience. The President has been kind enough to refer to the paper I propose to give you, as "Electricity in America in the year 1884;" but I would rather, after having thought more about it, that it be called "A Visit to Canada and the United States in the year 1884."