PHYSICS: C. BARUS 
549 
With the object of gaining some insight into the remarkable behavior 
at low pressure a new apparatus (no. Ill) was constructed with the glass 
plates spaced by a rectangular frame made of square brass tubing. The 
inside distance between plates was here 1.3 cm.; but in other respects 
it closely resembled the wood frame specified. The results with this 
metal case, however, differed totally from those of the other. In the 
morning of a bright day, there was usually marked repulsion between 
M and m, which changed gradually into an attraction at the close of 
the day. The repulsion was often so strong that the ends of the needle 
were pushed up into contact with the glass plates, to which position 
they returned whenever removed by tapping. 
It was found, however, that the needle could be immediately freed by 
exhaustion of the case. In other words the repulsions passed continu- 
ously into attractions which were here at their maximum at the highest 
Fig. 2 
exhaustions. The behavior of the metal case was thus the reverse of 
that of the wood case. In the former exhaustion removed a repulsion; 
in the latter, an attraction. It is difficult to assign a reason for this as 
there are three forces in contention: viz., gravitation and the radiant 
forces of the case (static) and of the external mass M. One is tempted 
to contrast the non-conducting wood with the conducting metal. The 
greater narrowness of the frame of the latter, however, gives the forces 
due to temperature distributions an advantage. In one respect the 
exhausted metal case has shown marked superiority; at a definite high 
vacuum, the excursions of the needle on any day are without drift; 
they are nevertheless variable on successive days. It is thus also im- 
probable that this vacuum excursion corresponds to the gravitational 
attraction, so that an adequately trustworthy excursion is yet in arrears. 
4. Record of the vacuum needle. — To exhibit these relations more 
clearly I have constructed figure 2, which contains a record of mean 
