254 
Proceedings of the Royal Society of Edinburgh. [Sess. 
XII. — On an Electrically Controlled Thermostat and other 
Apparatus for the Accurate Determination of the Electro- 
lytic Conductivity of Highly Conducting Solutions. By 
John Gibson, Ph.D., and G. E. Gibson, B.Sc. 
(MS. received November 19, 1909. Read June 22, 1908.) 
Kohlrausch and others have recently published investigations which 
show that the electrolytic conductivity of dilute solutions of inorganic salts 
may be determined with a maximum error of two or three in ten thousand. 
Hitherto such accuracy has not been attained with highly conducting 
concentrated solutions. With such solutions different difficulties are en- 
countered from those met with in dilute solutions. Temperature variations 
originating outside the cell, the heating effect of the current within the cell, 
and polarisation are sources of error which are particularly troublesome in 
the case of highly conducting solutions. 
By means of the electrically controlled thermostat shown in fig. 1 the 
temperature of the bath can be kept constant to about one thousandth of a 
degree centigrade.* The temperature variations are observed through a 
reading telescope on a Beckmann thermometer graduated to one-hundredth 
of a degree centigrade. When the thermostat is working properly the 
thermometer suspended in the bath remains perfectly steady for hours at 
a time. On one occasion no visible movement of the mercury was observed 
during four days. The whole apparatus has been in nearly constant use 
during the last four years. 
The cylindrical copper tank is painted white inside, and on the outside 
is covered with thick felt. It has a capacity of about 100 litres. 
The stirrer, shown in fig. 1, is of the screw propeller type, and has a 
diameter about one-third of the diameter of the tank. It is driven by an 
electromotor, and rotated so that the water circulates upwards at the centre 
and downwards at the sides. The lower end of the vertical brass shaft $ is 
rounded off so as to run in a foot-bearing of gun-metal fixed to the bottom of 
the tank. The upper end runs in a bearing in a horizontal cross-bar, not 
shown in fig. 1, and is reduced from \ inch diameter to J inch so as to form a 
* It is interesting to note that so long ago as 1865 Kohlrauscli described an apparatus 
for controlling temperature electrically. At that time the use of the electric current for 
technical purposes was quite undeveloped, and the appliances now so familiar were unknown. 
Poggendorf Annalen, Bd. cxxv. p. 126. 
