Intelligence and Miscellaneous Articles. 515 
until ten or twelve minutes after making the circuit; from this 
maximum point the strength of the current gradually diminishes. 
Fig. 2 shows the action of the same battery with ten ohms external 
resistance. Under these conditions we find at the instant of 
making circuit a strong current, which rapidly diminishes within 
the first five minutes to one sixth of its first strength. 
Knowing the distance of the galvanometer from the sensitive 
paper, the strength of the current may be calculated by measuring 
the distance between the two lines at any instant, and proceeding 
as with an ordinary galvanometer and scale. 
From a comparison of the two figures the electromotive force 
may be determined by Ohm’s law, if the distance between the lines 
is measured at the instant the current is made. Then knowing 
the electromotive force, current, and external resistance, we can 
readily find the internal resistance. This resistance will be the 
liquid resistance of the cell only for the moment that the circuit is 
made, for afterwards the variation in electromotive force due to 
polarization, and the change in resistance of the liquid due to 
electrolytic action will combine to cause changes. Since, however, 
the changes in electromotive force due to polarization are much 
more rapid in their action than the changes in battery resistance, a 
very small error will be introduced if we compare points near each 
other on those parts of the curve in which the variation in current 
is greatest: during the small fraction of a minute that is taken, 
the change in battery-resistance will be infinitesimal and may be 
neglected. 
We have selected these photographs as an example of the large 
variations that some batteries present, and the consequent useful- 
ness of some such way of studying their action. From measure- 
ments upon these photographic charts the variations in electro- 
motive force and internal resistance can be studied by obtaining 
such charts under different conditions of external resistance. It 
is evident that the same photographic method can be employed 
to study the swing of the needle of a short coil-galvanometer, 
which indicates the gradual heating of a thermopile. In this 
way the conduction of heat along a bar could be studied.— 
Silliman’s American Journal, May 1885. 
Jefferson Physical Laboratory. 
ON AN INSTRUMENT RESEMBLING THE SEXTANT, BY WHICH ANGLES 
WITH THE HORIZON CAN BE MEASURED. BY E. H. AMAGAT. 
Imagine’a sextant of which the optical axis of the telescope, 
instead of being oblique in reference to the fixed mirror, which I 
shall call B, is perpendicular to it. Arrange the movable mirror A 
so that its axis of rotation coincides with one of its edges, and cuts 
_ the optical axis. Suppose that the mirror B, instead of being fixed, 
is movable about an axis parallel to the plane of the graduated 
circle, and perpendicular to the optical axis; in these conditions, 
