Ans. It is a good plan, when the machine has been thoroughly cleaned and all connections made secure, to occasionally test the insulation of the different parts. If a record be kept of these tests, any deterioration of the insulation can at once be detected, localized and remedied before it has become sufficiently bad to cause a breakdown. As a means of protecting the machine from any moisture, dirt, etc., while standing idle, it is advisable to cover it with a suitable waterproof cover. CHAPTER XXX COUPLING OF DYNAMOS Series and Parallel Connections.--When it is necessary to generate a large and variable amount of electrical energy, as must be done in central generating stations, apart from the question of liability to breakdown, it is neither economical nor desirable that the whole of the energy should be furnished from a single dynamo. Since the efficiency of a dynamo is dependent upon its output at any moment, or the load at which it is worked (the efficiency varying from about 95 per cent. at full load to 80 per cent. at half load), it is advisable in order to secure the greatest economy in working, to operate any dynamo as near full load as possible. Under the above circumstances, when the whole of the output is generated by a single dynamo this can evidently not be effected, for the load will naturally fluctuate up and down during the working hours, as the lamps, motors, etc., are switched into and out of circuit; hence, although the dynamo may be working at full load during a certain portion of the day, at other times it may probably be working below half load, and therefore the efficiency and economy in working in such an arrangement is very low. Ques. How is maximum efficiency secured with variable load? Ans. It is usual to divide up the generating plant into a number of units, varying in size, so that as the load increases, it can either be shifted to machines of larger size, or when it exceeds the capacity of the largest dynamo, the output of one can be added to that of another, and thus the dynamos actually at work at any moment can be operated as nearly as possible at full load. Ques. What should be noted with respect to connecting one dynamo to another? Ans. It is necessary to take certain precautions (as later explained) in order that the other dynamos may not be affected by the change, and that they may work satisfactorily together. Ques. What are the two methods of coupling dynamos? Ans. They are connected in series, or in parallel. In coupling dynamos in series, the current capacity of the plant is kept at a constant value, while the output is increased in proportion to the pressures of the machines in circuit. When connected in parallel, the pressures of all the machines are kept at a constant value, while the output of the plant is increased in proportion to the current capacities of the machines in circuit.