ANALYTICAL EDITION
180
abrader and stock 1 with the U. S. abrader do show a slight trend with the temperature but this is so slight that it may be ignored.
Remarks In this investigation study of the effect of humidity during testing was omitted in the belief that during such a short time the humidity could not have an appreciable effect. Furthermore, there is a rise in temperature a t the surface being abraded, which would make it very difficult to control the humidity. While the tests were being made a record of the relative humidity of the cabinet was kept, but this seemed to make no discernible difference in the results. The relative humidity of the cabinet varied over a considerable range (from 18 per cent to 68 per cent) during these tests, but any differences observed varied in direction and, since it is within the range of experimental error, may be ignored.
Conclusions This study shows that in determining resistance to abrasion the temperature of the room or cabinet should be controlled within *lo C. in order to avoid significant errors in the results from this source. Below are shown the percentage differences per degree Centigrade obtained with each of the stocks. It must be remembered that this probably holds only over the range of temperatures studied and might change rapidly outside of this range. As the temperature changes from 15' C. to 35" C., the resistance to abrasion changes per degree Centigrade in the following manner: STOCK
u. s.
ABRADER Per cent
GRASSELLI ABRADER Per cent 4-1.40
1
+1.26
?
+0.40
+0.8
6
-1.8
-0.8
+o.so
VOI. 1, No. 3
either with the raw or vulcanized stock. Where laboratories are equipped to condition raw and vulcanized stock for stressstrain tests, it appears that it would be advisable also to condition samples for determining resistance to abrasion. It would a t least tend to produce more nearly uniform results by eliminating possible sources of small errors. As the relative humidity during exposure of raw stock increases from 10 per cent to 100 per cent, the resistance to abrasion per 1 per cent relative humidity changes roughly in the following manner: STOCK
1 4 5 6
u.s.
GRASSELLI ABRADER Per cent -0.03 -0.03 -0.01 -0.067
ABRADER Per cent +0.056
+O.OQ +0.09 +0.04
As these variations are small compared with the experimental error, it is evident that if the relative humidity does not vary over too wide a range its effect may be neglected. The temperature of storing the cured samples while maintaining a constant relative humidity has a negligible effect as in the case of relative humidity. As the temperature increases the resistance to abrasion per degree changes in the following manner: STOCK
-0.15 -0.15 +0.15 -0.03
-0.4 -0,025
6
W. A. GIBBONS,
Chairman J. M. BIERRR
E. R. RRIDGEWATSR
In the light of the present investigation there will be no great error in results caused by differences in relative humidity
GRASSELLI ABRADER Per cent
+O. 125 -0.3
1 4 5
+o. 1
u. s.
ABRADER Per cent
D. F. CRANOR C. R. PARIC
J. W. SCHADE N.A.SHEPARD A. A. S0MERVnI.E K. J. SOULE F. E. RUPERT, Research Associate
Electrode Holder for Arc Spectrum Analysis' Laurence L. Quill and Pierce W. Selwood UNIVERSITY OF ILLINOIS,URBANA, ILL.
HE electrode holder described herein was designed for use in the inorganic division of the Department of Chemistry at the University of Illinois, where routine examination of the arc spectra of the rarer elements is carried on. The holder consists of two horizontal arms mounted on an insulating fiber cylinder capable of rotating about its axis, the whole being adjustable up or down by means of a vertical screw running through the main support and supplied with a large hard-rubber handle. Both horizontal bars are thus adjustable together, while the lower bar has also ai1 independent vertical movement through a rack and pinion with two hardrubber handles. As the lower carbon electrode containing the element under examination is made positive, it burns away rapidly and requires frequent adjustment. The electrodes are held in place by means of steel springs over slots in the horizontal bars. I n practice one pair of electrodes is generally made of carbon while the other is of iron, which is used as a reference standard. It is possible then to bring either the carbon or the iron arcs into the axis of collimation of the spectrograph simply by rotating the cylindrical fiber support about its own axis. The whole apparatus is mounted on an alberene base and is about 15 inches (38 em.) high. 1
Received January 17,1929.
The holder has been found to facilitate the work for which
it was designed and should be useful for any other spectrographic study involving the juxtaposition of two arc spectra
on the same plate. It was made by A. E. Wood, mechanician for the Department of Chemistry.
