Edmond Becquerel himself explains this phenomenon and his findings in the 1842 publication “Mémoire sur le rayonnement chimique qui accompagne la lumière solaire et la lumière électrique”. He said the important fact discovered by him was that certain rays, which are incapable of carrying out a primary action on paper, are very suitable for continuing this action.
The Becquerel effect works also with silver bromide on paper; Becquerel actually did most of his tests with silver bromide. He announced that he had succeeded with papers impregnated with bromide or chloride of silver, and with plates of iodized silver.
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