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XXI.

ATMOSPHERIC ELECTRICITY.

BY ALEXANDER MCADIE AND AUSTIN L. MCRAE.

Communicated May 13, 1885.

By direction of the Chief Signal Officer, observations on atmospheric electricity were begun at Harvard College, Cambridge, Mass., under the supervision of Professor John Trowbridge, June 1, 1884.

By permission of the Chief Signal Officer the following abstract is taken from a report upon the apparatus used, and upon the observations from June 1, 1884, to April 30, 1885.

ELECTROMETERS.

A Thomson quadrant electrometer No. 26, and a Clifton modification of the Thomson, were originally used. A full description of the former can be found in the British Association Report for 1867, and also among the reprinted papers of Sir Wm. Thomson on "Electrostatics and Magnetism," Paper No. XX. The Clifton instrument is a modified form of the Thomson, designed for greater sensitiveness and of less complicated construction. It is not very generally known, and a brief description of it may therefore be given. The essential parts are four large brass or brass gilded quadrants, supported on glass rods of about 10 cm. length. A bifilar suspension carries an aluminium needle, corrugated and shaped like the figure eight. The length of the suspension is about 15 cm. A platinum wire from the needle dips into a glass vessel containing pure sulphuric acid, and coated on the outside and bottom with tinfoil. In the bottom of the case of the instrument, a circular opening is cut, of diameter sufficient to allow the removal of the glass vessel and the metal base on which it rests. In the upper part of the case is placed a small Thomson replenisher. The air within the case is kept dry by small open glass cups containing sulphuric acid. The needle is charged by means of a platinum wire imbedded in a gutta-percha rod, passing through the side of the case, and dipping into the sulphuric acid.

The instrument, as thus constructed, was found to be extremely sensitive, and admirably adapted to detect the smallest difference of potential of any two bodies, but for a long and continued series of observations, or single experiments of long duration, it was found unserviceable. The electric field of the quadrants was not sufficiently protected from external electrical influences. Two sides of the case only were coated on their interior surfaces with tinfoil, but the theory of the instrument demands, as far as possible, a complete shielding from external electrical influences. The great delicacy of the suspension is possibly the cause of the most serious defect, viz. a shifting of the zero point. The needle would never return exactly to its initial position. The difference was often great enough to give on a scale distant a meter from the mirror, a deflection of a centimeter. In the course of an hour a change in the position of the zero, when all the quadrants were connected, of from two to five or more millimeters would occur. These changes were in part due, no doubt, to a loss in the charge of the needle. To remedy the first defect, new suspending fibres were inserted without effect. To remedy the change due to dissipation of the charge, the vessel jar was paraffined around the edge, and for a time better results were obtained, though still faulty. The glass rods supporting the quadrants were several times taken out, washed with alcohol, dried, paraffined around the edges, and replaced. Other parts of the instrument, in which it was thought the fault might lie, were also carefully cleaned, and where needed provided with better insulation.

To obviate the difficulties met with in using the Clifton, the instrument described in the following sections was designed by Professor Trowbridge, and made at Boston, Mass. It is essentially the Clifton, so modified as to retain its great sensitiveness without having the defects mentioned. It was also desired to have an electrometer of more convenient arrangement than any of the forms now in use, -one in which the different parts should be amply protected from external influences, and yet be easily accessible for examination. The instrument devised has two compartments. In the upper compartment are the quadrants, needle, and suspending apparatus. In the lower compartment is the glass jar, with the arrangement for charging the needle. The upper compartment consists of a wooden case, 25 cm. high and 20 cm. square. On the top and back of the case are tightfitting brass doors, the one at the top being 12 cm. square, the door at the back being 16 cm. high and 12 cm. wide. When open, these doors allow easy access to the suspending frame, and when shut, form VOL. XX. (N. S. XII.)

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part of the metallic shield, covering and perfectly protecting the needle and quadrants from external electrical influences and also air currents. Circular brass windows 7 cm. in diameter, encased in brass and inserted in three sides of the case, allow inspection from without, of the needle, and also the passage of the beam of light to and from the mirror.

The bottom of the compartment consists of a brass plate of about 5 mm. thickness. From this plate rise, at alternate corners, two brass rods of 23 cm. height, supporting a cross-beam 20 cm. in length and 5 mm. in diameter. Fastened by a screw to this beam is a cross piece of 3.5 cm. length, supporting two suspending rollers. One roller has a screw movement to or from a central point exactly over the centre of suspension. The end of the roller is enlarged and grooved. One end of the suspending fibre is fastened to the roller by insertion through a small eye-hole in the shaft, and then made to pass in the enlarged groove. The other roller consists of a like brass shaft with an enlarged groove, in which the other end of the suspension fibre runs. This roller has not, however, a screw movement, and turning the screw head in this case simply raises or lowers the needle without change of position of the points of suspension. The whole suspension may be raised or lowered by movement of the screwhead attached to the supporting cross-beam. The plane of suspension may be altered by movement of the cross-bar, which is pivoted on the end of the screw passing through the cross-beam. This suspension is much simpler, equally sensitive with the best arrangements in other instruments, and in case of accident easily repaired. The length of the suspension is about 9 cm. Only one long fibre is employed. The platinum wire carrying the needle is hooked at its upper extremity, and by this means attached to the fibre. The weight of the needle is sufficient to insure a symmetrical suspension, without extra adjustment. If two separate fibres are used, and attached to a small cross-piece on the platinum wire, it will be necessary to test the symmetry of the suspension. The single fibre, however, allows a symmetrical suspension and with the arrangements employed allows easy and accurate adjustment. In the instrument constructed there is no error of position of the zero point. After the greatest deflection when short-circuited, the position of the spot of light on the ground-glass scale is exactly that of its initial position. Six months' constant use of the instrument has not necessitated the use of any correction for the position of the zero.

The needle is made of aluminium about 10 cm. in length, and at

its broadest parts about 2 cm. in width. The supporting platinum rod terminates in a small half-loop, just below the quadrants. To this loop a very fine platinum wire is attached, supporting a light lead paddle in the sulphuric acid of the glass jar. The quadrants are made of polished brass, the circle of which they are sections having a diameter of 15 cm. They are mounted on glass tubes, made of the best white glass, 5 cm. in height, and 1 cm. in diameter, and mounted on gutta-percha. Through the inside of the tubes run insulated wires imbedded in rubber, connected at the upper end with the quadrants, and at the lower end passing through the brass base plate to bindingscrews in the walls of the lower compartment.

One of the quadrants can be slid out, the supporting rod being inserted in a brass plate moving in a groove cut in the base plate. A spring plate with a small projecting knob, fitting into the notches of the base plate, keeps this quadrant perpendicular and firm wherever it may be placed.

The lower compartment is 23 cm. high and 20 cm. square. In it is placed a glass jar of 15 cm. diameter and 10 cm. depth. The vessel is tinfoiled on the outside and bottom, and rests on a circular brass plate which can be either elevated or lowered several inches. The back of the compartment is hinged, and when opened allows full inspection. A platinum wire imbedded in gutta-percha passes through the side of the glass vessel about a centimeter from its upper edge. This wire dips into the acid of the jar.

The instrument, thus constructed, has been in constant use during the past six months, and, requiring but little attention, has proved itself very well adapted for work of this nature.

MULTIPLE QUADRANT ELECTROMETER.

With a view to the construction of a portable electrometer sufficiently sensitive and accurate, the following instrument was designed by Professor Trowbridge and Mr. McAdie, and built according to their plans by the Western Electric Company of Boston. An exterior wooden case 30 cm. high and 12 cm. square, contains four compounded quadrants, a compound needle, and the suspending and charging arrangements. The outer case rests on a brass plate with the proper levelling arrangements, and is divided into three compartments, lettered A, B, and C. Each of these has one side at least hinged, so as to open and allow easy access to the interior. At the bottom of the front side of compartment A, a semicircular glass case 2 cm. in height projects. The bottom inside surface of this is mirrored in order to

eliminate errors of parallax in reading the position of a fine aluminium index playing over it. This uppermost compartment contains the suspension apparatus and the long light aluminium index arm. The suspension is as previously described. The aluminium pointer is carried by the platinum wire which supports the needle, but is insulated from it. A small concave mirror is also attached to the platinum wire, so that, if desired, the instrument can be employed with lamp and scale as a reflecting instrument. In the middle compartment B, four brass quadrants are mounted on flint-glass tubes of 4 cm. in length and 1 cm. in diameter. Each quadrant is compounded of four single quadrants. The dividing partitions fit into slots cut in the back plate, and are removable at pleasure. They are held in an exact horizontal position by means of small screws. The needle is made of aluminium, and is also of a compound type, being made of four or more single needles, connected and so arranged as to move between the quadrant sections. The interior surface of compartment B is completely tinfoiled. The third compartment contains a glass jar tinfoiled on the outer side and in connection with the ground. Through the side of the glass vessel is led a platinum wire encased in hard rubber. The deflection of the needle is recorded by the movement of the aluminium pointer. In the instrument constructed, when one set of quadrants is connected with the ground, the other to the positive pole of a Daniell cell, and the needle connected with the positive pole of a Beetz battery of 200 cells (described below), the movement of the index hand is perceptible to the unaided eye. On a scale distant 70 cm. from the mirror this deflection is nearly 2 cm. The length of the suspension is about 4 cm. Increasing the potential of the needle increases the sensibility of the instrument. If, instead of the method generally employed, we connect one set of quadrants with the positive pole of a battery of a number of cells connected in series, and the negative pole to the other set of quadrants and the needle connected with the body whose potential is to be determined, we obtain greater sensitiveness. The deflection obtained has then to be compared directly with the deflection given by a Daniell cell.

Connected in this manner, our electrometer gave a movement of the index hand, for a Daniell cell, of several degrees, or, with the mirror and scale, a deflection of about 4 cm.

For getting a continuous record, this form of electrometer is more easily adaptable than the others. It is also obvious that, aside from the difficulty and uncertainty of photography, an electrometer for successful use in meteorological work must be of such a nature

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