Thus it happens that the London-Paris telephone works better than was expected.  The nature of M is probably equivalent to about 0.0075 [phi] per mile, and therefore K should be also about 0.0075 [phi] instead of 0.0156 [phi] per mile.  This helpful action of mutual induction is present in all long circuits, and it is the reason why we were able to speak to Brussels and even to Marseilles.  It also appears in every metallic loop, and vitiates the measurements of electromagnetic inertia and of capacity of loops.  Thus, if we measure the capacity of a loop as compared with a single wire, the amount per mile may be 50 per cent. greater than it ought to be; while if we measure the capacity of one branch of a circuit under the conditions of the London-Paris telephone line, it may be 50 per cent. less than it ought to be.  This effect of M is shown by the dotted line in Fig. 1.

Telephonic currents—­that is, currents induced in the secondary wire of an induction coil due to the variation of microphonic currents in the primary wire—­are not alternating currents.  They do not follow the constant periodic law, and they are not true harmonic sine functions of the time.  The microphonic currents are intermittent or pulsatory, and always flow in the same direction.  The secondary currents are also always of the same sign, as are the currents in a Ruhmkorff coil, and as are the currents in high vacua with which Crookes has made us so familiar.  Moreover, the frequency of these currents is a very variable quantity, not only due to the various tones of voices, but to the various styles of articulation.  Hence the laws of periodic alternate currents following the sine function of the time fail when we come to consider microphones and telephones.  It is important to bear this in mind, for nearly everything that has hitherto been written on the subject assumes that telegraphic currents follow the periodic sine law.  The currents derived from Bell’s original magneto-transmitters are alternate, and comply more nearly with the law.  The difference between them and microphones is at once perceptible.  Muffling and disturbance due to the presence of electromagnetic inertia become evident, which are absent with microphones.  I tested this between London and St. Margaret’s, and found the effect most marked.

7. Lightning.—­A metallic telephone circuit may have a static charge induced upon it by a thunder cloud, as shown in Fig. 6.  Such a charge is an electric strain which is released when the charged cloud flashes into the earth or into a neighboring cloud.  If there be electromagnetic inertia present, the charge will surge backward and forward through the circuit until it dies out.  If there be no E.M.F. present it will cease suddenly, and neutrality will be attained at once.  Telephone circuits indicate the operation by peculiar and characteristic sounds.  An iron wire circuit produces a long swish or sigh, but a copper wire circuit like the Paris-London telephone emits a short, sharp report, like the crack of a pistol, which is sometimes startling, and has created fear, but there is no danger or liability to shock.  Indeed, the start has more than once thrown the listener off his stool, and has led to the belief that he was knocked down by lightning.