MODERN electrical measurements are conspicuous for the fact that the frequency of alternating currents can be measured to a very high order of accuracy. In order to provide a standard of frequency for scientific and technical workers, the National Physical Laboratory undertakes the emission of two types of frequency of reference from its own radio transmitting station. One of these is in the form of a modulation frequency of one kilocycle per second superimposed on a carrier wave of 396 kilocycles per second, while the other is a simple carrier wave of frequency 1,780 kilocycles per second. The programme incorporating the first frequency is emitted on the second Tuesday of each month at 10.40-12.00 G.M.T., while the second is emitted on the first Tuesday in March, June, September and December at 2LOO-22.00 G.M.T. The modulation frequency employed in the first programme is derived from one or other of two oscillators, which are maintained in continuous operation at the National Physical Laboratory. One of the oscillators is an electrically maintained tuning fork vibrating at its natural frequency of 1 kilocycle per second; while the other is a quartz ring oscillator generating a frequency of 20 kilocycles per second, with the necessary attachment for selecting the required frequency of one twentieth of this value. In both cases, the accuracy with which the frequency is maintained is one or two parts in 10 million, but during each emission the exact frequency is measured at the Laboratory, and the correct value is announced at the end of the programme. For the second standard frequency emission, a separate quartz crystal oscillator is employed; in this case, no correction is announced, the frequency emitted being accurate to within one part in a million. A revised programme giving full details of these transmissions can be obtained on application to the Director, National Physical Laboratory, Teddington, Middlesex.