DEPARTMENT OF TERRESTRIAL MAGNETISM. 331 



radium collectors, and the positive pole of the charging battery is connected ta 

 B instead of to earth, as in the usual arrangement. The negative pole of the 

 battery is, of course, connected to the collecting wire, A. In the determination 

 of the activity curve of the wire, the latter is wound on a frame and placed in 

 an ionization chamber as in the Elster and Geitel arrangement. The central rod 

 of the ionization chamber is connected, however, to the fiber of a sensitive 

 single-fiber electroscope of the Einthoven type and earthed. The outer cover 

 of the ionization chamber is insulated and kept charged to 150 volts by Kriiger 

 batteries. On releasing the fiber from earth it starts to move at a much more 

 rapid rate than is obtained in the usual method, where the outer case of 

 the ionization chamber is earthed and the central rod of the chamber is 

 charged to a potential far different from that of the earth, necessitating the 

 use of an electroscope of a much less sensitive type, A discussion is given of 

 certain points on the relation between the activity collected by the wire and 

 the radioactive content of the atmosphere. It is not always convenient to 

 deduce the actual amount of active material in the air, but it is suggested that 

 in place of the usual Elster and Geitel unit, which depends upon the particular 

 apparatus used, a more fundamental unit should be employed. A quantity 

 which may conveniently be recorded as a measure of the activity of the air is. 



eVC6t 



where C is the measured capacity of the portion of the collecting wire which is 



placed in the ionization chamber, V is the potential of the charging battery, 



8v 

 K is the capacity of the ionization chamber, — is the initial rate of alteration 



ot 



of potential of the electroscope when the wire is placed in the ionization 

 chamber, and e is the electronic charge. 



The third portion of the paper describes a piece of apparatus suitable for 

 obtaining relative values of the potential gradient. A tube, about 8 feet long, 

 is arranged so that it can be turned about its axis. A wire, insulated from it, 

 is stretched along its axis and connected to brass pieces fixed by ebonite plugs 

 in the ends of the tube. One of the brass pieces carries an arm about 50 cm. 

 long. The arm is perpendicular to the axis of the tube and carries at its end 

 a disk. The brass piece at the other end of the wire is connected to a special 

 key. In one position this key earths the insulated system and an electroscope 

 at the same time. On turning the tube slightly the insulated system and the 

 electroscope become disconnected from earth and from each other. On 

 turning the tube 180° from its initital position the insulated system again 

 becomes connected to the electroscope. It is obvious that unless the tube is 

 perpendicular to the equipotential surfaces, the electroscope will show a. 

 deflection, and the change of its potential will be proportional to the potential 

 gradient. The instrument was designed specially for work at sea and in this 

 respect possesses two chief advantages. In the first place, good insulation is. 

 not vitally important, as is the case with the usual collectors, since it is only 

 necessary for the insulated system to show no leak while it is being 

 turned through 180°. In the second place, the method enables the 

 potential gradient to be measured at an instant, and consequently readings 

 may always be taken at one position of tilt of the vessel during a rough sea. 

 This avoids uncertainty in the meaning to be attached to the results, owing to 

 the dependence of the quantity measured by the instrument on the shape of 

 the surface bounded by the ship and the sea. It is, of course, necessary to 

 standardize the instrument by comparisons Avith measurements made over a, 

 flat surface and free from disturbing influences. 



