686 



the middle); the iiiits are pressed against the ebonite by screws. 

 Besides, the lid k of the ionisation cylinder ƒ is attached to the 

 ebonite plate; for this purpose Ihe ebonite plate is perforated in six 

 places, so that from the top countersnnk screws can be driven into 

 the lid ; they will penetrate the lid only half way (Fig. 4 : 3). Six 

 clamping screws cause the upper edge / of the ionisation cylinder 

 to press closely against the lid. At tjie top the brass ionisation 

 cylinder is partly shdt off by the lid, partly by a small brass basin 

 consisting of two parts, with an opening in the centre for the 

 needle; bayonet closure unites these two parts, as well as the outer 

 part with the lid. The basin is to hold CaCl,, which has to 

 protect the ambroid from moisture; at the same time the needle 

 in the ionisation cylinder, except for the small space of air mentioned 

 above, is thus quite surrounded by metal, which is desirable for the 

 measurement of ionisation. The bottom of the cylinder consists of a 

 separate brass plate, pressed against the rin) of the cylinder by 

 clamping screws. As to the upper part of the apparatus, the chief 

 part certainly is the brass measuring cylinder c, the dimensions of 

 which are chosen in such a way as to cause the upper needle to 

 undergo a thorough damping during its motion. This cylinder tits 

 into a ring-shaped groove of the bottom plate and is provided with 

 a broad lim with two circular slits through which two screws with 

 notched heads fasten the measuring cylinder to the bottom plate. 

 This enables the measuring cylinder to move over a rather large 

 angle. (Cf. Fig. 2). In order to be able to check the state of things 

 inside the cylinder and to see whether the upper needle is in the 

 right position, there are in the walls of c, diametrically opposed, 

 two oval openings, covered with celluloid, which correspond with 

 tw^o openings of the same size in a second brass outer cylinder, 

 which is revolvable, so that during the measurement the inner space 

 can be entirely shut off by metal. 



The bottom plate i of the measuring space c is fastened by- 

 screws to brass legs, which support the apparatus. These rest on a 

 triangular wooden base with levelling screws fitted with ebonite 

 insulating toes. 



To the lower needle h, which slides tightly into an excavation 

 oid,'A. definite position can be given with the help of a scale made 

 on the bottom plate of c. 



The upper needle consists of a small aluminium strip, 0.05 mm. 

 thick, and is fastened to a thin aluminium rod with mirror and 

 mirror-supporter; the suspension consists of a thin platinum Wollaston 

 wire. 



