barus.] CALIBRATION OF ELECTRICAL PYROMETERS. 99 
Strouhal and 1 Lave shown. 1 In a series of corresponding positions of 
these commutators, alternately opposite, the direct measurements would 
give 
• +J fl - a ^~ ~^E~ -=«25- + h] = a 3 ;etc. 
where an odd number of observations is made. Let M\ be the mean of 
the odd right-hand members, and 7J/ 2 the mean of the even right-hand 
members. Then 
^=^(M l + M 2 ). 
In the case of small electro-motive forces this elimination is essential. 
Standards of electromotive force. — Inasmuch as all measurements are 
based upon the constancy of the double Daniell E, it is obvious that 
the value of this electro-motive force will have to be frequently tested. 
This can be done very simply and with accuracy by replacing the 
thermo-couple by a Latimer-Clarke or other standard element and pro- 
ceeding with the measurements as usual. It is to effect this compen- 
sation that the rheostat r must have a large resistance, 20,000 ohms, 
available. For thermo-electric work r = 0.1 to 500 ohms suffices. For 
a similar reason two Daniells are used instead of one. Currents are 
made only momentarily, and approximate values of r and R are always 
known. The constancy of my Latimer-Clark's cells has certainly been 
exemplary, and it was thus easily possible to reduce the Washington 
results to the older results obtained in New Haven in a way that estab- 
lished the general accordance of data beyond a doubt. In addition to 
the Latimer-Clark standards, I possessed for comparison a number of 
siphon Daniells, certain Beetz's dry Daniells, and a special form of nor- 
mal Daniell of my own which merits description. In this battery it is 
impossible, correct usage presupposed, for the copper sulphate to con- 
taminate the zinc. Zinc and zinc sulphate, copper and copper sulphate, 
are kept in separate bottles, and are only in electric contact during the 
few minutes of measurement. In Fig. 20 the Zu-Zn S0 4 bottle is on the 
left, the Cu-Cu S0 4 bottle on the right. Each bottle is provided with 
an h-shaped siphon of glass, the longer shank of which, a b c, is closed 
above by a rubber cap, a y and below by a cap of parchment paper tied 
on. The shorter shank d dips into a little vessel, A, containing Zn S0 4 
in solution. When not in use the siphon tube is nearly empty. Before 
using both tubes are rinsed thoroughly with Zn SG 4 solution by com- 
pressing and relieving the caps a. When clean they are filled in the 
same way with the Zn SG 4 of the dish A. After using, both siphons are 
emptied by working the cap a as usual, and the ends d are closed by 
special caps (not shown) to prevent evaporation. It is obvious that if 
the rinsing be properly done Cu S0 4 can not possibly get into the zinc 
flask to contaminate the metal. Zn therefore remains bright for years. 
I Bull. U. S, Geol. Survey, No. 14, p. 35, 
(753) 
