252 EXCITED RADIO-ACTIVITY [CH. 
the amount of free space in its neighbourhood. Excited radio- 
activity is also produced in water if exposed to the action of an 
emanating compound. 
166. Concentration of excited radio-activity on the 
negative electrode. When thorium or radium is placed in a 
closed vessel, the whole interior surface becomes strongly active. 
In a strong electric field, on the other hand, the writer found that 
the activity was confined entirely to the negative electrode. By 
suitable arrangements, the whole of the excited activity, which 
was previously distributed over the surface of the vessel, can be 
concentrated on a small negative electrode placed inside the vessel. 
An experimental arrangement for this purpose is shown in Fig. 48. 
Vv 
Thortum Oxide 
KX XK KK WK KK KK KKK KK KK KK KK KK KM KKK 

> Battery 
Fig. 48. 
The metal vessel V containing a large amount of thoria is con- 
nected to the positive pole of a battery of about 300 volts. The 
wire AB to be made active is fastened to a stouter rod BC, passing 
through an ebonite cork inside a short cylinder D, fixed in the side 
of the vessel. This rod is connected with the negative pole of the 
battery. In this way the wire AB is the only conductor exposed 
in the field with a negative charge, and it is found that the whole 
of the excited activity 1s concentrated upon it. 
In this way it is possible to make a short thin metal wire over 
10,000 times as active per unit surface as the thoria from which 
the excited activity is derived. In the same way, the excited 
activity due to radium can be concentrated mainly on the negative 
