Such incidents as these serve merely to illustrate the lighter moments that relieved the severe and arduous labors of the strenuous five years of the early storage-battery work of Edison and his associates. Difficulties there were a-plenty, but these are what Edison usually thrives on. As another coworker of this period says: "Edison seemed pleased when he used to run up against a serious difficulty. It would seem to stiffen his backbone and make him more prolific of new ideas. For a time I thought I was foolish to imagine such a thing, but I could never get away from the impression that he really appeared happy when he ran up against a serious snag." It would be out of the question in a book of this kind to follow Edison's trail in detail through the innumerable twists and turns of his experimentation on the storage battery, for they would fill a big volume. The reader may imagine how extensive they were from the reply of one of his laboratory assistants, who, when asked how many experiments were made on the storage battery since the year 1900, replied: "Goodness only knows! We used to number our experiments consecutively from one to ten thousand, and when we got up to ten thousand we turned back to one and ran up to ten thousand again, and so on. We ran through several series--I don't know how many, and have lost track of them now, but it was not far from fifty thousand." The mechanical problems in devising this battery were numerous and intricate, but the greatest difficulty that Edison had to overcome was the proper preparation of nickel hydrate for the positive and iron oxide for the negative plate. He found that comparatively little was known by manufacturing chemists about these compounds. Hence it became necessary for him to establish his own chemical works and put them in charge of men specially trained by himself. After an intense struggle with these problems, lasting over several years, the storage battery was at length completed and put on the market. The public was ready for it and there was a rapid sale. Continuous tests of the battery were carried on at the laboratory, as well as practical and heavy tests in automobiles, which were kept running constantly over all kinds of roads under Edison's directions. After these tests had been going on for some time the results showed that occasionally a cell here and there would fall short in capacity. This did not suit Edison. He was determined to make his storage battery a complete success, and after careful thought decided to shut down until he had overcome the trouble. The customers were satisfied and wanted to buy more batteries, but he was not satisfied and would sell no more until he had made the battery perfect.