MOUNTING A DEMONSTRATION GALVANOMETER.

BY F. R. GORTON,

Michigan State Normal College, Ypsilanti, Michigan.

In the January number of this magazine the author showed a method for mounting a Nernst lamp filament for use in the laboratory. It has proved to be so convenient for many purposes that it may not be amiss to show a

special form in which this luminant may be used with the demonstration galvanometer. The chief advantage offered lies in the fact that the brilliancy of the light is such as to make a perfectly visible image on a scale in full daylight. The disadvantage offered in the form previously described is due to the necessity of heating the filament until its conductivity is above a certain value before the current is able to bring it to its maximum brilliancy. In the form shown in Fig. I the

A

B

3

initial heating of the filament is done by the current of electricity through the heater coils B which soon bring the filament A to the required temperature. The holder, together with the heater coils, filament, and ballast resistance can be procured of electric lighting companies, or the Nernst Lamp Company of Chicago. Figure I will sugest a simple mounting for this holder which is made by drilling three holes through a block of hard wood. This ame block is provided with a vertical hole for receiving a rod o which it may be clamped at any desired point. A current rough 1 and 2 passes through the heater coils B, while that through 1 and 3 goes through

The connections are shown in Fig 2 in which C and D are the ends to be connected with the mains whose P. D. must be given on ordering the parts. The ballast may be placed anywhere between A and D, as at R, and the key inserted at K. Having the key open, the current may be turned on wherever the connections are made with the lighting circuit; as in a lamp socket, for example. Practically no current will flow until K is closed, when the current is then carried by the coils B which heat the filament. In about 30 seconds the filament will begin to glow with extreme brightness, at which time K should be opened.

H

Figure 3 shows a plan for mounting the lecture-room D'Arsonval galvanometer G on the ledge above the blackboard. To the under side of the shelf is attached an arm extending 30 in. from its edge. At the outer end of this armı is a vertical rod on which the lamp holder (Fig. 1) is clamped. At L, 27 in. from the glower of the lamp, is placed a convex lens of 20 in. focal length. About 6 feet

from the galvanometer is hung from the ceiling by the hooks HH the scale of ground glass. The parts are so adjusted that the light from the luminant passes through the lens upon the mirror M which reflects the image to the scale at I. Keeping the lamp and lens a little below the perpendicular to the mirror enables the reflected light to pass entirely above the lens to any part of the scale. The key and ballast may be placed at any convenient point on the wall and the terminals of the galvanometer brought over the lecture table at a suitable height where the binding-posts on a small block of wood or porcelain enable the instructor to make connections easily.

LAWS OF FALLING BODIES.

A unique device for illustrating the laws of uniformly accelerated motion, composition of motions, trajectories, etc., has been recently brought out by John C. Packard, Science Master, High School, Brookline, Mass. A steel ball, one inch in diameter, placed at the top of an inclined plane of wood or of plate glass, is

given an initial velocity at right angles to the slope of the plane, by being compelled to roll down an auxiliary incline behind a ledge, and is then allowed to roll down the plane. The path of the ball being the resultant of uniform and uniformly accelerated motion is, of course, a parabola. To secure a tracing of this curve, a piece of co-ordinate paper is secured to the plane, and a piece of soft carbon paper of the same size is placed over it. The ball, in rolling over the transfer sheet, leaves its mark upon the co-ordinate paper. Any number of duplicates can be made by repeating the experiment under precisely similar conditions, or the curve may be varied at will by changing the incline of the principal plane, the auxiliary plane, or both. Measurements made upon the curve thus traced will readily reveal the laws of uniformly accelerated motion, and the fundamental principles underlying the phenomena of falling bodies.

A pendulum attachment, not shown in the illustration, may be used to determine the value of the time interval in seconds, if desired.-Scientific American.

SOME PRACTICAL APPLICATIONS OF THE ELECTRIC

FURNACE

BY FRANK G. TAYLOR,

High School, Oregon, Illinois.

A very simple and serviceable electric furnace can be made with two ordinary fire brick. Along the center line of each, drill a semi-circular groove somewhat larger than the carbons you wish to use. For the lower brick make a cavity in the center about two inches square and one and one-half inches deep. In the center of the other brick make a dome shaped cavity about two inches in diameter. Place discarded electric light carbons in the grove of the lower brick. Fit the upper brick in place and the furnace is ready for the current. (See cut.)

The following are a few very simple but interesting experi

ments.

Place some small pieces of marble in the cavity of the lower brick. Close the furnace and turn on the current. Light fumes appear around the electrodes. Bring a lighted match to them and they burn with a pale bluish flame, showing the presence of carbon monoxide. The CO is probably formed by the union of the CO: with the carbon in the arc, as shown by the ejuations, CaCo Ca+CO2, CO:+C=2CO. In about ten minutes a bright red reflection may be seen on a sheet of paper held near the end of the electrodes, showing the presence of vaporized calcium in the arc. The top may now be removed. An examination shows that the marble has been transformed into quicklime which will slake with great intensity upon the addition of water.

2nd. Repeat the process using soft coal. Soon after the current is turned on dense fumes pour out around the electrodes. Apply a lighted match to the fumes. They burn with a bright flame, showing the presence and source of commercial illuminating gas. After the gas ceases to burn an opening of the furnace reveals that the soft coal has been transformed into a good grade of coke. By using a larger amount of coal and collecting the fumes over water the presence of ammonia can be shown.

3rd. Grind together in a mortar the quicklime and coke that has been formed and repeat the process using this mixture. This time light fumes will be given off which will burn with a pale flame, indicating that carbon monoxide is being formed. After a few minutes an examination of the cavity will show a somewhat crystalline mass between the electrodes. If a portion of this mass is dropped into a test-tube containing water a rapid evolution of gas with the unmistakable odor of acetylene is observed. A lighted match brought to the mouth of the test-tube shows the characteristic acetylene flame. No further evidence is needed to show that the coke and quicklime have united, under the intense. heat, to form calcium carbide.

4th. A mixture of sand and coke, with a little salt, will yield bright blue crystals of carborundum, a compound that has never yet been found in Nature. Between the electrodes may also be found silica in the "spun glass" form.

5th. By drawing each end of one of the electrodes across a sheet of paper it will be shown that the hard gritty carbon has been transformed into a soft graphite.

With little care many of the uncommon metals may be reduced from their oxides. Graphite crucibles may be used in this work. Copper and other common metals are easily vaporized and the vapors collected. Space forbids a description of these experi

ments.

The current to be used may have an almost unlimited range and yet give results. Either direct or alternating current may be used. The most satisfactory being a direct current of low voltage carrying 75 amperes, or more. Very satisfactory results for laboratory use can be obtained by tapping an ordinary incandescent light circuit carrying 110 volts and from 15 to 20 amperes. If an incandescent circuit is tapped care should be taken to use a rheostat to prevent the burning out of fuses.

If the bricks do not fit closely together cover the lower brick with a thin layer of powdered air-slaked lime. In this way all light can be kept in the interior, except along the electrodes.

This space should be left open for the escane of gases.