( 276 ) 
this a layer of 1 em. of yellow HgO is placed in aa and of red 
MgO in bb. The layer of oxide is made thick in order to prevent 
movement of the 
F050) 650° 5 surface of the mer- 
cury electrodes. 
According to 
the measurements of 
HELMHOLTZ, irreg- 
ular vibration of 
the surface of the 
electrodes may give 
k rise to differences 
“Ur of potential between 
| them, which must, 
of course, be avoided. 
The two limbs and 
the connecting piece 
Hoo yell. cc with the tap k 
were filled with a so- 
lution of potassium 
hydroxide (about 15 
Fig. 1. percent), the tap re- 
maining closed meanwhile. In this way the two oxides remain 
each in its own compartment whilst the cel! is being prepared. 
After filling them, the two limbs are immediately closed by india- 
rubber stoppers kk and kk. A small Anscutrz thermometer, (graduated 
in 1/;°) passes through each, the air being allowed to escape by 
means of capillary tubes / and /. These tubes are closed at once 
after filling in order to prevent entrance of water into the element 
when it is placed in the thermostat. 
The use of the thermometers is necessary, because small differences 
of temperature between the liquids or the mercury in the two limbs 
of the element might give rise to thermocurrents which would 
interfere with the measurement of the very small differences ot 
potential which are being dealt with. 
KOH sol. KOH sol, 
| 
He 0 red —— 
6. The arrangement and course of the measurements may be 
somewhat fully described, since the value of the results is very 
closely connected with them. The elements, also the standard cells, 
were kept in the dark during the whole course of the research. The 
influence of light on red and yellow mercuric oxide is not yet suffi- 
ciently studied to permit one to decide whether this agency would 
have any effect on the EK. M. F. which is to be measured. 
