Excerpt from Dynamo, Motor and Switchboard Circuits for Electrical Engineers: A Practical Book Dealing With the Subject of Direct, Alternating and Polyphase CurrentsThe frequencies of practical machines vary between about 25 to 120 cycles per second.Now, each cycle consists of two alternations or reversals, therefore the number of alternations is equal to twice the number of cycles. In the above case, the number of alternations or reversals per second 2 x 40 80.About the PublisherForgotten Books publishes hundreds of thousands of rare and classic books. Find more at www.forgottenbooks.comThis book is a reproduction of an important historical work. Forgotten Books uses state-of-the-art technology to digitally reconstruct the work, preserving the original format whilst repairing imperfections present in the aged copy. In rare cases, an imperfection in the original, such as a blemish or missing page, may be replicated in our edition. We do, however, repair the vast majority of imperfections successfully, any imperfections that remain are intentionally left to preserve the state of such historical works.