protecting the system from any injury due to an excessive current of electricity.

The small wires leading to each lamp from the main wires must be very thoroughly insulated, and, if separated or broken, no attempt made to join them while the current is in the main wires.

DYNAMO MACHINES.

Dynamo machines must be located in dry places, not exposed to flyings or easily combustible material, and insulated upon wood foundations. They must be provided with devices capable of controlling any changes in the quantity of the current; and, if these governors are not automatic, a competent person must be in attendance near the machine whenever it is in operation.

Each machine must be used with complete wire circuit; and connections of wires with pipes, or the use of ground circuits in any other method, is absolutely prohibited.

The whole system must be kept insulated, and tested every day for ground connections at ample time before lighting to remedy faults of insulation, if they are discovered.

Preference is given for switches constructed with a lapping connection, so that no electric arc can be formed at the switch when it is changed; otherwise the stands of switches, where powerful currents are used, must be made of stoneware, glass, slate, or some incombustible substance which will withstand the heat of the arc when the switch is changed.

250

ABSTRACTS.

LORD RAYLEIGH-ON PROFESSOR HIMSTEDT'S DETERMINATION OF THE OHM.

(Phil. Mag., Vol. 21, No. 128, Jan., 1886, pp. 10–13.)

The leading feature in the method of Professor Himstedt is the use of a commutator or separator, by which the make-and break-induced currents are dissociated, one or the other passing in a stream at equal small intervals of time through a galvanometer. The commutator makes contact by means of mercury cups, and Lord Rayleigh points out how very capricious and uncertain such contacts are in their action. A second point raised relates to the measurement by the galvanometer of a series of induced currents, each of short duration. In Himstedt's method the needle of the galvanometer stands in an oblique position, and we have to consider whether the axial magnetisation does not alter under the action of a force having a sensible axial component. There is yet a third point which may be open to objection, viz., the determination of K, the constant expressing the number of turns of the primary of the induction coil per unit of length. Apparently the value of K is arrived at by dividing the whole number of turns, 2,864, by the total measured length, 135.125 cm. ; but in so doing it is assumed tacitly that the turns are all perfectly uniform, or that only the mean value is required, both of which assumptions may be questioned.

Professor E. J. HOUSTON-PHOTOGRAPHY BY A LIGHTNING

FLASH.

(Journal Franklin Institute, Vol. 91, No. 3, March, 1886, pp. 221-22.)

The view taken was that of the surrounding country near Philadelphia at 7 p.m. on the 29th Oct., 1885. The night was excessively dark, with much wind and rain. The camera was placed at an open window, with the slide drawn, and immediately after the flash the slide was returned. The plate-a highly sensitive gelatine film-was reversed, and exposed a second time.

From the behaviour of the plates on development, it was estimated that the actinic effect of the light was equivalent to that obtained by an exposure ofth of a second in bright sunlight. It is possible, therefore, that the average severe flash lasts a longer time than is generally supposed. The author has himself observed motion of foliage under illumination from a flash of lightning; and this observation is confirmed by the photographs above mentioned, in which it is unmistakably evident that the foliage has moved during the time of exposure. It would appear, therefore, that photography may be used advantageously to determine the duration of a flash of lightning.

EDISON'S SYSTEM OF TELEGRAPHING WITH TRAINS IN MOTION. (Scientific American, Vol. 54, No. 8, Feb. 20, 1886.)

The receiving apparatus at both the car end and the fixed end of the line is a telephone. The sending apparatus is also similar at both ends, and consists of an interruptor or vibrating tongue driven by an independent battery, and making 500 vibrations per minute; this vibrator is in circuit with the line battery, an ordinary Morse key, and the primary of an induction coil. The secondary of the induction coil on the car is in connection with the tin covering the entire roof of one or more cars; the secondary coil at the fixed station is in connection either with condensers or with other induction coils, which in turn are in connection with the ordinary line wires by the side of the track. Suppose a message to be sent from the fixed station to the car. The vibrator is always working, but till the Morse key is put down no current passes. The message is sent by the ordinary Morse signal, only instead of a continuous current being sent to line each time, it is an alternating one; this induces a current in the secondary coil, and through it the condensers, for example, are charged alternately. The charge of the condensers is propagated through the line wires with which they are in connection, and influences the tin roof of the car, and ultimately the telephone by which the signals are read.

CORNU and POTIER-EXPERIMENTAL VERIFICATION OF VERDET'S LAW IN DIRECTIONS NEAR THE PERPENDICULARS TO THE MAGNETIC LINES OF FORCE.

(Comptes Rendus, Vol. 102, No. 8, pp. 385-91.)

Former verifications have only been carried out for the directions where the magnetic rotary power is considerable, and the authors thought it desirable to go further-to the directions where the rotation becomes nil.

Faraday having discovered that the magnetic rotation w becomes nil in the direction perpendicular to the lines of force, and changes sign when the pencil of light passes from one side to the other of this direction, it follows that the angle is an uneven function of the angle 8 which the pencil makes with the direction at right angles to the lines of force.

The question is if the term 8 really exists. From the existence of this terri follows the consequence that the two surfaces cut each other at a finite angle proportional to b.

If Verdet's law is exact, this term does exist, for the law may be written

The chief experimental difficulties were to obtain a very intense field, which should also be uniform. The former was overcome by the use of pecu. liarly-shaped electro-magnets, powerfully excited, between which was sus

pended a tube containing a saturated solution of iodide of mercury and iodide of potassium, which possesses three times the rotary power of bisulphide of carbon. The second condition was much more difficult to fulfil, but it was at the same time not so essential for the verification. It was, in fact, sufficient to prove the equality of the rotations produced by two columns of unequal length, but the extremities of which were respectively situated on the same two equipotential surfaces. The experiment was carried out by placing in the horizontal symmetrical plane of the electro-magnets a tube, having at its centre a cross branch onetenth as long, filled with the solution above mentioned. The results obtained verified Verdet's law so far as the apparatus was capable of doing so, and any small differences may be attributed to non-uniformity of the magnetic field.

A. LEDUC-DEVIATION OF THE EQUIPOTENTIAL LINES AND VARIATION OF THE RESISTANCE OF BISMUTH IN A MAGNETIC FIELD.

(Journal de Physique, Vol. 5, Mar., 1886, pp. 116–23.)

A strip of bismuth, 54 mm. long, 32 mm, wide, and 0-0233 mm. thick, is immersed in a vessel of distilled water, which is placed between the poles of a powerful electro-magnet, excited by a current of 38 ampères. The current passes through the strip of bismuth in the direction of its length, while two points in the other two sides of the strip are brought to the same potential. As the resistance varies not only with the strength of the field, but also with the temperature, it was necessary to make two series of observations. The effect of the magnetic field is shown by the following figures:-Magnetic field equal 0, R 0-0333; magnetic field equal 5,000 units, R 0.0354; magnetic field equal 10,000 units, R 0·0387—an increase of 16 per cent. The experiments showed that the deviation of the equipotential lines is independent of the current passing, but that it depends on the strength of the field and on the temperature, and that it does not exceed 5o. The deviation may be expressed by the following formula, where M is the strength of the magnetic field and t the temperature:—

=

C = 303 × 10-15

(L'Electricien, Vol. 10, Nos. 148 and 149, pp. 97-100 and 113-15.)

This installation is made on the Gaulard and Gibbs system of secondary generators or transformers. At the central station are two compound steam

engines, driving two Siemens alternate-current machines with their two exciters; the 30 bobbins of the alternate-current machines are coupled up so as to give a current of 32 ampères, with an E.M.F. of 2,500 volts, the number of alternations being 16,500 per minute. From these machines the current goes out into the main conductors through the town. The secondary generators, which are a special form of induction coil, are arranged in parallel circuit on these main conductors. There are four groups of these secondary generators, all similar. Each group consists of two generators of two columns, with a closed magnetic circuit for each two columns. Each column has a separate primary and secondary circuit, built up of discs of copper, the centre of the discs being cut out to allow of the passage of an iron core. From the secondary generators the service lines go to each subscriber's house or shop. As all the transformers are arranged in parallel, it is necessary to maintain a constant difference of potential at the terminals of the machines; this is done by altering their exciting current. By means of a special apparatus the regulation may be made automatic, the apparatus putting in or taking out resistance as necessary.

E. GERARD-LOCALISATION OF FAULTS BY MEANS OF A

TELEPHONE.

(La Lumière Electrique, Vol. 19, No. 9, pp. 408-10.)

One end of the line is insulated; the other end is put in connection with a vibrator, by means of which alternating currents can be sent into the line from a battery, the other pole of which is to earth. If now a person walks along the line carrying in one hand a coil with an iron core, and in the other a telephone in connection with the coil, the inductive action of the current in the line will cause the telephone to sound; but the sound will stop as soon as the fault is reached. The experiment has been practically made by the author, and the fault located exactly. The method is especially useful in the case of underground electric light cables, where, owing to their very low resistance, the ordinary methods of testing would fail to localise the fault exactly, and it would become necessary to open a considerable length of ground. The coil used should have as many turns as possible, and its resistance should approximate to that of the telephone used; the core should consist, preferably, of a bundle of iron wires. One of the chief advantages claimed by the author is that the method can be carried out successfully by an unskilled person, and he believes it to be capable of considerable development.

F. and W. KOHLRAUSCH-THE ELECTRO-CHEMICAL EQUIVALENT OF SILVER; AND AN EXPERIMENTAL PROOF OF THE EARTH'S MAGNETIC INTENSITY.

(Annalen der Physik und Chemie, B. 27, Pt. 1, No. 1, 1886, pp. 1-59.) This is a detailed account of two very elaborate series of experiments carried out in the years 1881 and 1883, with the view of determining very exactly the