Chap. XII Of Electrometers
We have already seen that it is a general law of electricity, that similar electricities repel, and that dissimilar electricities attract each other.—On this law all electrometers are constructed. In fact the cork balls, which have been mentioned are electrometers, and exhibit at once the most important phenomena for the explanation or ascertaining of which the instruments which bear this name are constructed. Still it is of use to see the application which may be made of this general principle. It is applied to ascertain the quantity of the electric fluid collected either in a prime conductor or a coated jar; and also the state of the atmosphere in regard to electricity, and the character of that electricity at any particular time and place.
The instruments by which these purposes are effected we shall now shortly describe.
To ascertain the quantity of electricity in a prime conductor or jar, an electrometer the most easily constructed and of the most general use has been invented by Mr. Henley—called the quadrant electrometer.—Of this we have given a representation in the frontispiece, (letter X.)
It consists of a perpendicular stem formed at the top like a ball, and at the lower end with a screw, by which it is fastened to the prime conductor. A graduated semicircle of ivory, horn or stiff paper, is fixed near the uppermost end of the stem. A moveable index, made of a slender piece of hickory, extends from the centre of the graduated semicircle a little distance beyond its circumference, having a small ball of cork or pith at its lower extremity.
When the conductor or jar is not electrified, the index is parallel to the stem, but when it is electrified the index recedes more or less, according to the degree of the electrization, which is marked on the graduated circle.
A simple atmospheric electrometer was constructed by Mr. Cavallo in the following manner.—
To the end of a common fishing rod, he affixed a slender glass tube covered with sealing wax, and having a cork at its end, from which two cork or pith balls were suspended by hempen strings. From the other end of the rod proceeded a flaxen or hempen twine a little longer than the whole rod and tube, with a pin attached to it, which was stuck into the cork at the extremity of the glass tube, for the purpose of taking off the insulation. The twine, to prevent its falling when the pin was pulled out of the cork, was attached to the rod, by a small string, running from it and meeting the rod at a little distance from the glass tube.
To use this instrument, let the pin be pushed into the cork. Then, holding the rod by the extremity farthest from the cork balls, project it out, from a window in the upper part of the house, into the air, raising the end of the rod to which the balls are appended, so as to make an angle of 50° or 60°, with the horizon.—After having kept it in this situation a few seconds, by pulling the twine, detach the pin from the cork.—This leaves the electrometer insulated, and electrified with an electricity contrary to that of the atmosphere. Now draw the instrument into the room and you may examine the quality of the electricity, by applying the knob of a phial positively charged to one of the balls; if the ball is attracted by the knob it is negatively electrified—if repelled, positively electrified.
The satisfaction arising from these experiments is sometimes abated, from the circumstance that the quantity of electricity obtained in this way, is so small that its quality cannot be ascertained. To remedy this inconvenience Cavallo and Nicholson, have invented machines which they denominate doublers or multipliers of electricity. But the structure of these machines is complex and delicate, and the explanation of them is long, and not easily understood without the aid of plates. Our epitome therefore does not admit of inserting them. Those who may choose to pursue the subject we refer to the writers above mentioned.
To prevent the inconvenience arising from wind and rain in the use of the atmospheric electrometer, the following device has been used by Mr. Cavallo.—Take a glass vessel open at top and bottom—cement it at bottom to a convenient piece of wood—let the upper part be tapering like the neck of a phial, and cement into it a glass tube, extending a little above and a little below the neck of the larger vessel. Cover the tube with sealing wax, both within and without the neck of the vessel, so as to give it the appearance of one body. Into this tube cement a brass wire extending a very little below the bottom of the tube, and flattened at the lower end so as to be perforated with two small holes. Through these holes insert two flaxen threads, or two very fine silver wires, with small balls of cork or pith at the end of them, and touching each other:—if wires are used they should be suspended by small rings at the top, that they may act more easily. Let the top of the brass wire screw into a brass cover on the top of the whole vessel, which cover will not only secure the vessel against rain, but serve as a conductor to a very slightly electrified atmosphere—conveying the fluid, first to the wire, and by means of that to the balls, which will exhibit, within the vessel, the state of electricity collected from the atmosphere. There should be two narrow slips of tin foil stuck to the inside of the glass vessel, and communicating with the wooden bottom, which will serve to carry off that electricity which, when the corks touch the glass is communicated to it, and which, if accumulated, would disturb the free motion of the corks.
An useful alteration of this electrometer was made by Mr. Bennet. It consists of slips of gold leaf or silver leaf, instead of the corks suspended by threads or wires. These slips of leaf are to be suspended from the cover of a cylindrical vessel, and hanging within it. The slips of leaf are to be about two and an half inches long. This electrometer is the most sensible instrument of the kind, manifesting in an unequivocal manner very small quantities of electricity. But this instrument is not as portable and easily managed as the other.—If very fine threads, stiffened with glue, be used without any balls, they will be found nearly as sensible as the gold leaf.