In the course of his investigations - he carried them on for a long time - Galvani was astonished to observe that some of his specimens, which he had hung on to an iron railing by means of brass hooks, sometimes fell to twitching even when the sky was quite clear and there was no sign of thunder. His natural conclusion was that this must be due to hitherto unnoticed electrical changes in the atmosphere. Observations maintained for hours every day, however, led to no conclusive result; when twitchings did occur it was only with some of the specimens, and even then there was no discoverable cause. Then it happened one day that Galvani, 'tired out with fruitless watching', took hold of one of the brass hooks by which the specimens were hung, and pressed it more strongly than usual against the iron railing. Immediately a twitching took place. 'I was almost at the point of ascribing the occurrence to atmospheric electricity,' Galvani tells us. All the same he took one of the specimens, a frog, into his laboratory and there subjected it to similar conditions by putting it on an iron plate, and pressing against this with the hook that was stuck through its spinal cord. Immediately the twitching occurred again. He tried with other metals and, for checking purposes, with non-metals as well. With some ingenuity he fixed up an arrangement, rather like that of an electric bell, whereby the limbs in contracting broke contact and in relaxing restored it, and so he managed to keep the frog in continuous rhythmical movement.