[Illustration: Figs. 732 to 735.--Various starting resistances. The type of resistance used in motor starting rheostats of small size consists usually of tinned iron wire wound on asbestos tubes, as shown in fig. 732, the tubes being firmly supported by porcelain nipples, the ends of which fit into holes in the top and bottom of the enclosing case. In starters of larger size, cast metal grids, as shown in fig. 733, are used. In addition to these types of resistance, some forms of starter are equipped with what is known as "unit" type resistance. In this form, the resistance is built up of a number of separate sections, or units, which are connected to form the complete starting or regulating resistance as the case may be. A single unit consists of a moulded core of vitreous material upon which is wound the resistance wire, as shown in fig. 734. The surface of the unit is then coated with a special cement and baked. By this method the resistance material is protected from mechanical injury and is also made proof against moisture and other conditions which sometimes affect the ordinary type of resistance. In addition to units coated with cement only, there are still other types of units, as in fig. 735, which are provided with a sheet metal covering around the cement, as a further precaution against injury. Each of the various types of resistance described possesses certain characteristics not shared by the others, the use of any particular type being largely governed by conditions of service.] Ques. What is the difference between a starting box and a speed regulator? Ans. Motor starting rheostats or "starting boxes," are designed to start a motor and bring it gradually from rest to full speed. They are _not_ intended to regulate the speed and must not be used for such purpose. _Failure to observe this caution will result in burning out the resistance which, in a motor starter, is sufficient to carry the current for a limited time only_, whereas in the case of speed regulators sufficient resistance is provided to carry the full load current continuously. [Illustration: Fig. 736.--View of Cutler-Hammer starter with slate front removed showing open wire coil resistance. The type of resistance here used consists of tinned iron wire wound on asbestos tubes. The bottom of the casing is perforated to secure ventilation.] Ques. For what kinds of service are speed regulators used? Ans. In cases when the speed must be varied, as in traction motors, organ blowers, machine tool drive, etc. Ques. How long does it take to start a motor? Ans. Usually from five to ten seconds. Ques. How is the starting lever operated? Ans. It is moved progressively from contact to contact, pausing long enough on each contact for the motor to accelerate its speed before passing to the next. Ques. What are the conditions at starting in a series motor?