This work by Philipp Lenard* clearly originated from the lectures on experimental physics the author had held over a number of decades as professor in Heidelberg.[¹]They were famous, and justifiably so. Whoever heard the author lecture, even if it were only once, could not avoid coming away with the impression that he is a great personality gifted with a deep understanding of diverse aspects of physics and instilled with a sincere love for nature. This book seeks to preserve as much of this as possible for posterity. Thus naturally it cannot exactly be a modern textbook of physics, which fact might become more apparent in the volumes to follow the present one under discussion.