E. 21. Wellisch — Mobilities of Ions in Air. 585 



aperture 3 cma in diameter. In the upper compartment was a 

 copper plug on which was a deposit of polonium which had 

 been kindly prepared for me by Professor Boltwood ; this plug- 

 was placed near the circumference of the partition and extreme 

 care was taken that all the radiation was confined to the upper 

 compartment. A circular electrode A (4 cms in diameter) was 

 situated about 3 cras above the aperture and was in metallic con- 

 tact with the case of the vessel. The lower compartment con- 

 tained the gauze electrode insulated by a thin ebonite ring 

 from the partition. The gauze consisted of brass wire -77 mm 

 thick with 6-J meshes to the cm. Two cms. below the gauze 

 was the electrode e connected to the electrometer ; surround- 

 ing this electrode was a guard ring which was metallically con- 

 nected to a guard screen (W) consisting of a gauze cylinder 

 mounted on a circular plate. Both the ring and the screen 

 were connected to earth by means of the guard tube shown in 

 the diagram. The object of the screen was mainly to prevent 

 disturbing effects which might arise as a result of the rapid 

 changes in potential undergone by the case of the vessel. The 

 insulation consisted throughout of ebonite. 



Four brass tubes admitted of connection between the vessel 

 and other apparatus ; through one of the tubes the air was 

 admitted after passage through glass tubes containing P 2 5 , 

 CaCl 2 , and glass and cotton wool. Pressures above 2 cms were 

 read on a mercury gauge one limb of which was evacuated ; 

 pressures below 2 cms were ascertained by means of a McLeod 

 gauge. Both a Fleuss and a Topler pump were used for pur- 

 poses of evacuation. By means of the cylinder and reservoir 

 containing mercury of which a sketch is given in the diagram 

 the air was passed several times through a tube containing P 2 5 ; 

 in this way any traces of water vapor could be removed before 

 observations were made. 



The Commutator. — The commutation of potential was 

 effected by means of the principle employed by Langevin in 

 his experiments. This is illustrated in fig. 2. The two poten- 

 tials V 1 and -Y 2 under consideration were connected across the 

 terminals of a large metal resistance R. The com mutating 

 device consisted of a circular brass disc with a number of 

 equally spaced slots cut at regular intervals along its periphery; 

 these slots were filled with vulcanized fiber which was made 

 flush with the curved surface of the disc. Brushes consisting 

 of bent copper strips were mounted at regular intervals on a 

 circular frame close to the disc. Mounted on the same shaft 

 as the commutating disc, and in metallic connection with it, 

 was a collector ring consisting of a brass disc 4-J cm in diameter. 

 The commutator was placed between that end of the resistance 

 connected to the gauze and the point whose potential was V, 



