No. III. ON THE EFFECT OF ARMATURES ON THE MAGNETIC STATE OF ELECTRO-MAGNETS.

By B. O. PEIRCE AND E. B. LEFavour.

M. JAMIN has lately shown that the effect in providing a steel magnet with an armature consists merely in a redistribution of magnetism, but not in an increase. The following experiments were instituted, to determine what was the effect of armatures of electro-magnets on their magnetic state. The method of experimenting was to slip a coil of fine wire over the electro-magnet, which was provided with a scale, and to measure by the swing of the needle of a reflecting galvanometer the induction currents which arise on making and breaking the circuit of the electro-magnet. The first experiments were made with a straight electro-magnet, 19 cm. long, 1.5 cm. wide. The core consisted of a bundle of fine iron wires, the ends of which were filed in one plane, upon which the armature, which consisted of a piece of iron 6 cm. long, 1.5 cm. wide, rested. It was found that the mass of this armature made no difference, as long as the end of the core of the electro-magnet was completely covered, and in close contact with the armature. The following table gives the results obtained. Only the currents produced by breaking the primary circuit are given.

When these results are represented by curves, they show that the magnetic state, in a straight electro-magnet without an armature,

increases from both ends towards the middle (which was shown to be the case by Jacobi and Lenz in 1844); and when the armature is applied, the curves diverge greatly near the pole upon which the armature is placed.

Our next experiments were tried with a solid horseshoe-shaped electro-magnet, the limbs of which were 12 cm. long, 2.5 cm. in diameter; the resistance of the electro-magnet was about .01 of an ohm and that of the induction coil 1 ohm.

The scale runs, increasing, from the bend of the electro-magnet to the pole of the limb on which the induction coil was slipped. Here there is a marked increase, resulting from the use of the armature, which is shown in a striking manner by expressing the above results as curves. The results of our experiments show:

1st. The application of an armature to one pole of a straight electromagnet results in an increase of the magnetic condition of the magnet.

2d. The application of an armature to both poles of a horseshoeshaped magnet results in a remarkable increase of its magnetic state, which increase is much the greatest near the armature.

No. IV. ON THE TIME OF DEMAGNETIZATION OF SOFT

IRON.

By W. C. HODGKINS AND J. H. JENNINGS.

THE following experiments were undertaken to determine the length of time that the core of an electro-magnet remained magnetic after the cessation of the magnetizing current. A chronograph, provided with two pens, was used to measure the intervals of time. The method adopted to obtain these results on paper was as follows: The lower pen of the chronograph, which was movable by a lever, worked by hand, was connected with the electro-magnet and with the battery which served to excite the electro-magnet. By pressing the lever down, the circuit of the electro-magnet was broken, and at the same instant the lower pen was moved upwards. The upper pen formed the armature of a small electro-magnet, and moved downward upon the passage of a current through the coils of this magnet. One end of the wire of this small electro-magnet was connected with the positive pole of a Bunsen cell, the negative pole of which was joined to a brass plate, which was placed immediately beneath the large electromagnet which was to be tested. The second wire from the small electro-magnet, which worked the upper pen of the chronograph, was connected with a small piece of soft iron which formed the armature of the large electro-magnet.

It will be seen that when no current was passing through the large electro-magnet, its armature would rest upon the brass plate immediately beneath it, and the circuit of the upper pen of the chronograph would be completed. On the other hand, upon the passage of a current, the armature would be raised, thus breaking the circuit of the

upper pen.

The method adopted was to pass the current through the coil of the large electro-magnet, start the chronograph, and then, by means of the lever on the lower pen, repeatedly break and make the circuit; thus making and breaking the circuit of the upper pen, and moving both pens at intervals, which represented the required time of demagnetization. The interval required to demagnetize the small electro-magnet, in the circuit of the upper pen, did not enter into the results, since this pen was used merely to denote the instant when its circuit was made. The time of demagnetization was determined in this manner, with a coil 220 mm. in diameter, and with cores successively 54 mm., 41 mm., and 29 mm. in diameter. The battery power varied from four cells

to ten cells of grove. The armature was a piece of soft iron, weighing 22 grammes. The thickness of the coil, which was equal to the lengths of the iron cores, was 65 mm. The wire of which it was composed was 3 mm. in diameter.

The results of these experiments are given in the following table:

Experiments were then made with horseshoe electro-magnets; but it was speedily found that the phenomenon of the adherence of the armature, after the breaking of the galvanic circuit entered, and the time of release of the armature, was practically infinite. With straight electro-magnets, the above experiments show that the magnetic strength sufficient to maintain an armature of constant weight at the respective poles, had a duration of .091 of a second, and appeared to be not sensibly affected by increase or diminution of the magnetic state of the core, beyond that requisite to barely sustain the weight of the

armature.

XIII.

CONTRIBUTIONS FROM THE PHYSICAL LABORATORY OF THE MASSACHUSETTS INSTITUTE OF TECHNOLOGY.

E. C. PICKERING, PROFESSOR OF PHYSICS.

No. II.-LIGHT TRANSMITTED BY ONE OR MORE PLATES OF GLASS.

BY W. W. JACQUES.

Read, April 13, 1875.

THE following experiments were made for the purpose of determining the percentage of light transmitted through 1, 2, ..., 10 plates of glass, normal to the direction of the light, and of one, four, and ten plates when i was 0o, 5o, ..., 65°.

The apparatus used consisted of a triangular frame, isosceles and right angled, having a periphery of 100 inches. A gas jet was placed at the right angle, and two mirrors were so placed at the other angles as to reflect the light from the jet along the hypothenuse, thus giving the effect of two equal sources of light 100 inches apart. The plates of glass were mounted on a graduated circle placed between the jet and one of the mirrors, and the light cut off was measured by a Bunsen disc, movable along the hypothenuse of the triangle. (See "Physical Manipulation," Expt. 67. Pickering.)

The plates used were of common 12 × 18 window glass, and were carefully cleaned with rotten-stone, and then dried by rubbing with chamois skin immediately before each experiment.

The experiments were made in a dark room, whose walls were painted black, and it was found that the reflection from a sheet of paper, or even from the clothes of the observer, was sufficient to prevent the accurate setting of the disc. The following tables give the results of the experiments; each number being the mean of four observations, and the probable error of a single observation being 0.42 of one per cent.

Table I. gives the percentage of light transmitted by 1, 2, ..., 10 plates when i 90°. The first column gives the number of plates, the

=