As has already been stated, the sensitiveness of a cell to light is increased by proper usage.  This increased sensitiveness is shown, not by a lowered resistance in light, but by an increased resistance in dark.  This change in the cells goes on, more or less rapidly, according as it is retarded or favored by the treatment it receives, until a maximum is reached, after which the resistance remains practically constant in both light and dark, and the cell is then “matured,” or finished.  The resistance in dark may now be 50 or even 100 times as high as when the cell was first made, yet, whenever exposed to sunlight it promptly shows the same resistance that it did in the beginning.  The various treatments, and even accidents, through which it has passed in the mean time, seem not to have stirred its molecular arrangement under the action of light, but to have expended their forces in modifying the positions which the molecules must normally assume in darkness.

Practical applications.—­There are many peculiarities of action occasionally found, and the causes of such actions are not always discernible.  In practice, I have been accustomed to find the peculiarities and weaknesses of each cell by trial, developing its strongest properties and avoiding its weaknesses, until, when the cell is finished, it has a definite and known character, and is fitted for certain uses and a certain line of treatment, which should not be departed from, as it will be at the risk of temporarily disabling it.  In consequence of the time and labor expended in making cells, in the small way, testing, repairing damages done during experiments, etc., the cost of the cells now is unavoidably rather high.  But if made in a commercial way, all this would be reduced to a system, and the cost would be small.  I may say here that I do not make cells for sale.

The applications or uses for these cells are almost innumerable, embracing every branch of electrical science, especially telegraphy, telephony, and electric lighting, but I refrain from naming them.  I may be permitted, however, to lay before you two applications, because they are of such general scientific interest.  The first is my

Photometer.—­The light to be measured is caused to shine upon a photo-electric current-generating cell, and the current thus produced flows through a galvano-metric coil in circuit, whose index indicates upon its scale the intensity of the light.  The scale may be calibrated by means of standard candles, and the deflections of the index will then give absolute readings showing the candle power of the light being tested.  Or, the current produced by that light and that produced by the standard candle may be compared, according to any of the known ways of arranging and comparing different lights—­the cell being lastly exposed alternately to the two lights, to see if the index gives exactly the same deflection with each light.