ART. VI.-The effect of Temperature upon Atmospheric Electricity; by HENRY GOLDMARK. (Contributions from the Physical Laboratory of Harvard College. No. 24.)

SIR WILLIAM THOMSON, by means of the different forms of electrometers, devised by himself, has investigated the electric potential of the atmosphere under varying conditions and in different localities. The effect of an increase of temperature upon the potential of the air he does not appear to have ascertained, and the following experiments were undertaken with a view of arriving at some conclusion upon this subject. From the nature of the case the measurements made were approximate, and the results reached qualitative rather than quantitative. To get the potential of the air at any point I used the water-dropping apparatus devised by Thomson. Its construction will be readily understood by reference to the sectional view given below.

It consists merely of a can of water A, insulated by standing upon a glass support, and discharging by the small pipe p through a fine nozzle. The insulation is made more perfect by drying the atmosphere around the insulating stem by means of small pieces of pumice stone, moistened with sulphuric acid and placed around its base.

The water breaking into drops from the nozzle assumes the potential of the air at the point and communicates it to the copper plate C, from which the insulated wire B leads to the electrometer. In order to investigate the effect of different temperatures upon the potential, it was necessary to have a limited volume of air upon which to experiment. To do this I enclosed the nozzle of the water-dropping tube and the copper plate in the interior of the cylindrical drum D. This drum was made of several layers of sheet iron, so arranged that the air, after being heated by three Bunsen burners below, would pass several times around the cylinder and so raise the temperature of the enclosed air, without otherwise affecting it. The water which dropped from the plate collected in the glass dish below from which it was drained out by a syphon, as fast as it fell.

To measure the potential I employed one of Thomson's quadrant electrometers, which was charged sometimes by a Holtz machine and sometimes by the Ruhmkorff coil. This instrument carries a small concave mirror which reflects spot of light upon a scale of ground-glass placed at the distance of about a meter. In order to avoid any influence which the observer's body near the instrument or conducting wires might make, I observed the deflections upon this scale by means of a telescope from the opposite end of the room.

The method of procedure was as follows: The opposite quadrants of the electrometer were connected with two of the four binding screws of a peculiar key constructed for the instru ment, while to the other two the wire coming from the measuring instrument and a wire leading to the earth were respectively attached. The potential of the air in the drum was then measured, the temperature being that of the air of the room. The result showed a very constant negative potential, varying but little in the successive days on which the experiment was made. The potential of the air of the room was also measured at the same time, and found to be the same as that of the enclosed air, thus proving that the drum had no effect upon the electric condition of its contents. The burners were then lighted, and the potential was, from time to time, measured after the temperature of the enclosed air, as observed by the thermometer t, had risen above that outside.

The change was by no means marked, but I did not on any occasion, notice any decrease of potential, but on the contrary a small but constant increase on raising the temperature. The following table gives some of the measurements made:

On extinguishing the burners and allowing the enclosed air to cool slowly, it retained the maximum potential it had reached on heating, even after it had regained its original low tempera

ture.

As a result of my experiments I arrived at the following conclusions:

1st. That, even a very considerable change of temperature, does not have any great or marked effect upon the electric potential of the air.

2d. That however a rise in temperature does produce a slight but constant increase in the potential.

ART. VII-A method of recording Articulate Vibrations by means of Photography; by E. W. BLAKE, Jr., Hazard Professor of Physics, Brown University.

THE extreme minuteness of the vibrations of the iron disc of the Bell telephone withdraws them from all ordinary methods of observation and measurement. A pointed wire fastened to the center of a ferrotype disc 2 inches in diameter, and moving on smoked glass, gave inches as the extreme amplitude of vibration under a powerful impulse of the voice, while sounds moderated to such a point as to be fairly articulate, were with difficulty detected by the movement which they communicated.

Animal membranes, possessing greater flexibility than the metal disc, seemed to promise better results, but the inertia of the attached wire, and the resistance offered by the smoked surface, become of importance, and throw doubt on the accuracy of the results obtained. Dr. Clarence J. Blake employs the human membrana tympani as a logograph,* and has obtained very beautiful and interesting tracings. I find by examination of some, which he kindly sent me, that the number of vibra tions as recorded falls considerably below the ordinary pitch of the voice, being in some cases as low as 80 per second.

The logograph described by W. H. Barlow, F.R.S., in a paper read before the Royal Society,t serves to record the varying pressures of the expelled air taken as a whole. With a single exception the diagrams give no suggestion of the musical character of the sounds. The width of the line drawn by a camel's hair brush and the slow movement of the paper would mask the minute vibrations even if the apparatus were otherwise adapted to showing them.

The opeioscope, invented by Professor A. E. Dolbear, consisting of a tense membrane, to the center of which a small mirror is attached, is well adapted to proving the existence of musical vibrations in human speech, but not to determining their character.

The phonograph of Mr. Edison records on tin-foil enough of the vocal elements to reproduce intelligible articulation. The minute indentations are therefore a record of great scientific value. In the hands of Mr. Fleeming Jenkin they promise to lead to valuable results in the analysis of vocal sounds. Dr. S. Th. Steing described in 1876 a method of photograph* Archives of Opthalmology and Otology, vol. v, No. 1, 1876. + Reprinted in the Popular Science Review, London, July, 1874. Nature, May 9th, 1878. Article on Phonograph by Mr. J. Ellis. § Poggendorff's Annalen, Bd. clix, S. 142.

ing the vibrations of tuning forks, strings, &c., by attaching to them plates of blackened mica punctured with small holes. A beam of sunlight passing through the hole strikes a sensitive plate moving with uniform velocity, and leaves a permanent record of the combined motions. Dr. Stein considers his method applicable to vocal sounds, but I cannot learn that he has ever attempted this application. My own experiments in that direction by Stein's method resulted in failure.

The object of this paper is to describe a method of obtaining photographs of minute vibrations on a magnified scale.

A plane mirror of steel, A, is supported by its axis in the metal frame B. The ends of the axis are conical, and carefully fitted into sockets in the ends of the screws C, C. On the back of the mirror is a slight projection D pierced by a small hole.

The vibrating disc, as hitherto employed, is a circular plate of ferrotype iron, 24 inches in diameter, screwed to the back of a telephone mouth-piece of the form

B

invented by Professor John Peirce, and now universally used. From the center of the back of this disc a stiff steel wire projects, the end of which is bent at a right angle. This wire serves to connect the vibrating disc with the mirror by hooking into the hole in D, as represented in the figure. The mirror frame and the vibrating disc are kept in a fixed relation to each other by a block of hard wood, to which both are firmly screwed. The mirror is set with its axis parallel, and its reflecting surface perpendicular, to the vibrating disc.

Back view of Mirror, actual size.

A heliostat sends a beam of sunlight horizontally through a small circular opening. This beam passes into a dark closet and at a distance of several feet from the circular opening falls upon the mirror above described placed with its axis inclined 45° to the horizon. The rays, reflected vertically downward, pass through a lens at whose focus they form an intensely luminous image of the circular opening.

A carriage moving smoothly on four wheels travels beneath the lens at such a distance that the sensitized plate laid upon it comes at the focus for actinic rays. A uniform velocity is given to the carriage by a string fastened to it and passing over a pulley. To this string a lead weight, just sufficient to balance friction, is permanently attached, while a supplemental weight acts at the beginning of motion and is removed just before the sensitized plate reaches the spot of light above described.

The velocity attained by the carriage is determined by placing a sheet of smoked glass upon it and letting it run under a

tuning fork (Ut 3—512 v. s.) provided with a pointed wire. In every case more than 200 vibrations were counted and measured, and careful comparisons made between the earlier and later ones, so as to be certain of the uniformity of the motion.

From the description it will be evident, that when the carriage alone is in motion a straight line will be photographed upon the plate. On speaking into the mouth-piece the disc is set in vibration, each movement causing change of angular position of the mirror, the reflected light moves through twice this angle, and the resulting photograph gives us the combination of its motion with that of the carriage. *

The general character of the curves obtained is shown in the accompanying figures, which are about one-half (0.56) the actual size of the originals. The reduction was accomplished by photography on the wood itself, so that the skill of the engraver was employed simply to follow the lines, which he has done with great fidelity.

The velocity of the carriage for the vowel-sounds was 21, for Brown University, 40, and for How do you do, 14 inches per second.

In the mathematical discussion of these curves the abscissas are measured by the known velocity of the carriage, and serve to determine the pitch, the ordinates represent the amplitude of vibration of the center of the disc, magnified 200 times in the photographs. The reduction of scale makes the magnifying in the wood-cuts only 112 times.

The ordinates are not strictly straight lines, but parts of the vertex of a parabola, and closely approximate to circular arcs whose radius is the focal length of the lens employed. In the figures given, the centers of curvature of these arcs is at the right hand.

With an ordinary tone of voice an amplitude of nearly an inch is obtained, implying a movement of the center of the disc of 005 inches as determined by actual measurement.

By varying the accelerating weight and its fall, any manageable velocity may be given to the carriage. Each syllable requires for its articulation about one-fourth of a second, hence

*The carriage should run from right to left. The negative (examined from the glass side), and prints taken from it, then give the syllables in their proper order, and show movements of the disc from the speaker by lines going from the obserThe arrangement of my dark room compelled me to make my carriage move from left to right; hence, in the figures given, forward positions of the disc are represented by the lower portions of the curves.

ver.

It can easily be shown that the reflected beam describes the envelope of a cone, whose apex has an angle of 90°, and whose axis is inclined 45°. The intersection of this cone with the horizontal plane gives the parabola. The lens employed transfers the apex to its own optical center. The ordinates may be made practically straight lines by placing the mirror with its axis vertical so as to reflect the beam almost directly back on its path, and having the sensitized plate move up and down in a vertical plane.