[Footnote 7:  No. 24 was measured with a bridge multiplier of 6 to 1.]

Cells which are sensitive to light improve by being used daily, and their sensitiveness becomes less if they are laid aside and not used for a considerable length of time, especially if allowed to become overheated.  They should be kept cool, and exposed to light frequently, whether they are used or not.

Mode of measuring cells.—­So great is the sensitiveness of these cells to external influences, that it is necessary to adopt some particular system in measuring their resistance and to adhere strictly to that system, as every change in the method of measurement produces a difference in the result, and the different measurements would not be comparable with each other.  The reason for this will be explained presently.

The system I have adopted is the Wheatstone’s bridge arrangement, with equal sides, never using multipliers except for some experimental purpose.  In each multiplier wire I have 500 ohms resistance.  When the bridge is balanced, one-half of the current flows through the cell and acts upon the selenium.  Between the bridge and the cell is a reversing switch, so that the current can be reversed through the cell without changing its course through the bridge.  A Bradley tangent galvanometer is used, employing the coil of 160 ohms resistance.  The Leclanche battery is exclusively used in measurements for comparison.

2. The kind of battery employed has a marked effect upon the sensitiveness to light, which is largely reduced or entirely destroyed when the bichromate battery is used.  The same cells again become extremely sensitive with the Leclanche battery.  We might expect that a change in the current employed would cause a change in the resistance of a cell, but it is not clear how or why it should affect the sensitiveness of selenium to light.

“If one kind of battery current destroys its sensitiveness, may we not suppose that another kind might increase its sensitiveness?  Although the Leclanche has operated well, some other may operate still better, and by its special fitness for use on selenium cells may intensify their actions, and so bring to light other properties yet unthought of.  Is not here a promising field for experiment, in testing the various forms of battery already known, or even devising some new form especially adapted to the needs and peculiarities of selenium cells?”

One year ago I made the foregoing suggestion in a paper on A New Form of Selenium Cell, presented before this Association at Minneapolis.  I am now at liberty to state that my photo-electric battery, presently to be described, marks an advance in the direction indicated.  The current from this battery increases the sensitiveness of the cells to light, and also to reversal of current.  One cell whose highest ratio in light was about 83 to 1, with the Leclanche battery, when measured with my battery gave a ratio of 120 to 1.  It seems to make the resistance of the cell both higher in dark and lower in sunlight than with the Leclanche battery.  But the field is yet open to others, for the discovery of a battery which may be still better for use with selenium cells.