2. In the absence of a convenient direct supply for a permanent installation, accumulators may be used after being charged elsewhere; but if much work is to be done, it is better, in our opinion, to instal some form of generating apparatus to provide a direct supply. This may be understood when we come to speak of the process of recharging accumulators, for, if a primary source be inconvenient, then the chances are that the accumulators will not be recharged so frequently or regularly as they ought to be in order to preserve their efficiency. Under such circumstances, also, transport of the accumulators renders this a troublesome and costly method for regular working. Here again, however, the use of accu- mulators may be combined with the installation of a primary source of supply, where that may be more conveniently or economically brought into action only at periodic times.

8. Where a supply of any nature is available inter- mittently, accumulators are eminently serviceable to render the energy available as it may be required.

For conversion of an unsuitable regular supply accumu- lators may prove of service, as in the method, for a time employed by us to utilise an alternating current, described on p. 44.

In most cases we think some of the other means suggested will prove more commendable, but each case must be con- sidered in relation to its own special conditions.

Usually, where the use of accumulators is indicated, it will be necessary for the X-ray worker to superintend their re- charging, and to see personally that they are maintained in a state of efficiency. This personal responsibility may be obviated if the cells be sent to an electrician, or to a generating- station, where expert attention may be expected; but such, as we have said, is a most inconvenient and costly mode of working. Further, it is very frequently under circumstances which render it impossible to obtain expert assistance that accumulators will be found most useful.

The operator being thus directly responsible, we have, on request, decided to enter more fully into the questions of charging and working accumulators than into most of the other details dealt with in this small work. For men in army or navy service we trust this will be especially useful.

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