You may perhaps find it interesting if we append a list of all the varieties of storages now used in electronic computers, with their comparative prices and performances. Before we do that, however, we must introduce a storage system which we have not previously mentioned: the disc storage, also known as the juke box. When you press the juke box buttons (which contain numbers and letters), the stock of records turns in obedience to this command until the pickup has selected the disc you desire. Disc storage functions in just the same way. The discs are coated with a magnetic layer, though, and carry nothing but information. A jointed arm with a reading head looks for the right discs, makes its way in between them and lifts off the information needed. The whole procedure takes place in a twinkling.
Magnetic tape is used for card indexes, catalogues, lists, dictionaries, large tables and accounts. Magnetic discs have the same uses as magnetic tape, but have smaller capacity. Magnetic drums are used for buffer storages, catalogues, intermediate results, programs and computations. Core storages are used as working storages, programs, for intermediate results, and accounts. Flip-flops are used for briefly needed data, registers, accumulators, and intermediate results.
We still have to explain the two ideas "bit volume" and "average capacity." The bit volume is the number of bits that can be accommodated in a cubic centimeter - about one-sixteenth of a cubic inch. Storages with a small bit volume swell the size of a computer. The "average capacity" on the other hand tells you the sizes of the storages built today in the categories concerned.
The next thing you need to know is how an electronic computer finds its way about in the jungle of storages, addresses and different ways of making calculation without falling prey to a nervous breakdown. You will read about this in the next chapter. It might almost be called a sad chapter. For unfortunately, left to its own resources, the electronic computer is in no condition to find its way about, as we shall see.