Most of the apparatus here illustrated and described may be constructed and used by any one having ordi nary mechanical skill. Simple and easily made devices have been chosen for physical demonstration. With scarcely an exception the experiments described were performed at the time of writing, to insure fullness of detail, and to avoid inaccuracies. The reader can therefore be assured, by following the instructions, success will be certain. Mathematics have been almost entirely excluded. The few problems presented are capable of arithmetical-solu tion. The importance of mathematical knowledge in all branches of science is fully recognized, but the majority of students have little taste for the intricacies of numbers. Faraday was an illustrious example of a scientific man without great mathematical proclivities. The late Clerk Maxwell, one of the most eminent mathematicians and electricians of the present century, said: A few experiments performed by himself will give the student a more intelligent interest in the subject, and will give him a more lively faith in the exactness and uniformity of nature, and in the inaccuracy and uncertainty of our observations, than any reading of books, or even witnessing elaborate experiments performed by professed men of science.