M usually limited? 27. What can you say about the commercial success of the step-by-step method? 28. State the principles of a lock-out party line. 29. For what purpose is a condenser placed in the receiver circuit of each station in the K.B. lock-out system? 30. How are the selecting relays in Roberts line restored to their normal position after a conversation is finished? 31. What are the objections against the Roberts system? REVIEW QUESTIONS ON THE SUBJECT OF TELEPHONY PAGES 287--315 * * * * * 1. What are electrical hazards? 2. When is th