the most convenient for measuring currents through gases.

Professor Dolezalek has more recently devised a form of electrometer which is exceedingly simple in construction and which may easily be made very much more sensitive than the older form of the instrument. Such an electrometer is shown in Fig. 5.

The chief modification consists in making the needle of paper coated with a thin film of silver so that it is exceedingly light. In this way it is found possible to do away with the sulphuric acid damping, and to depend on the resistance offered by the air to the motion of the needle to bring it to rest. The needle is suspended by a fine quartz fibre, so that the sensitiveness is greatly increased. It is a matter of no very great difficulty to get a deflection of the needle of 1 metre for 1 volt difference of potential between the quadrants, on a scale placed at a distance of 1 metre from the instrument, which is a sensitiveness exceedingly difficult to obtain with the ordinary pattern of instrument. The quadrants are supported on amber columns which insulate

very perfectly, and the whole instrument is mounted on a revolving table for convenience of adjustment.

USE OF ELECTROMETER TO MEASURE CURRENTS

IN GASES

The current through a gas is frequently measured between two parallel plates immersed

QUADRANT ELECTROMETER

A

B

K

EARTH

FIG. 6. Arrangement for measuring current through a gas between two parallel plates by quadrant electrometer.

in the gas (Fig. 6). One of the plates A is connected to one pair of quadrants of the electrometer, the other being connected to

earth. The plate B is connected to a battery of small secondary cells giving sufficient voltage to saturate the current between B and A. One pair of quadrants is kept permanently connected to earth, the other pair being connected to earth by the key K at the beginning of the experiment. On lifting K the current flowing from B to A through the gas causes the quadrants connected with A to charge up and the needle of the electrometer begins to move. The rate of motion of the needle gives a measure of the quantity of electricity passing through the gas in a given time, and therefore of the current. In practice it is best not to observe the needle while in motion, but to allow the current to flow from B to A for a definite time and then to remove the ionizing agent, or if this is impossible to disconnect the quadrants from A. The needle is allowed to come to rest, and the steady deflection gives a measure of the quantity of electricity which has passed through the gas in the given time.

In Fig. 7 is shown another form of apparatus which is frequently employed. The plate A

(Fig. 6) is replaced by the rod C (Fig. 7), and the second plate connected to the battery is replaced by a cylindrical vessel coaxial with the rod C. This vessel is connected to one pole of a battery of secondary cells, the other

FIG. 7.-Apparatus for measuring current between cylinder and co-axial rod immersed in ionized gas.

pole of which is connected with the earth. The vessel and rod are insulated from each other by the ebonite stopper R, through which passes a metallic tube T connected to earth. This serves to prevent any current flowing across the plug to the electrometer if the insulation of R is faulty.