The historical background of design included, of many different types of relays, not always known even to specialists, has been given much attention not only because it is interesting, but more important then that because of the frequent need for a display of expertise on the subject, enhancing the perception of competency of the specialist.

 In describing some of the complicated types of relays (for example electronic relays) the related issues of design and principles of operation of the relay components are discussed too (in our case vacuum, gas discharge and semiconductor devices), which allows the reader to better understand the principles of operation of the described relays, without having to refer to additional information sources.

 The book is written in a clear and easily understood language, without mathematical treatment, and includes numerous illustrations, making it attractive not only for specialists in relays, but also for a wide range of engineers, technicians and students interested in extending their knowledge in electric relays. Lecturers and university teachers will also find a lot of valuable material for their lectures in the book.