We had no sooner got the two telephone wires properly strung and attached to the hundred glass insulators when a thunder storm came up, and drove us back to the mill for shelter. Pretty soon the bell rang and we, supposing that some one at the cottage was trying to call, went to the instrument, but could get no response, nor could we make the bell ring. Lightning had sent an alternating current over the line which rang the bell, but the strength of the current was too great for our coils of fine wire and one of them was burned out, as we say. In other words, the wire had melted at the point where it offered the greatest resistance. The burned-out coil was replaced, and then we installed lightning arresters which were of two kinds. The first were simply fuses which were introduced into the line to protect it against any current too large for the apparatus to carry, and the second was a plate, _c_ (Fig. 172). These are to be found upon the top of the magneto cases. A wire is connected with _c_, and its other end is grounded by being connected with a piece of iron pipe which is driven deep into moist earth. [Illustration: Fig. 172] The plate _a b_ is inserted in the line, and the gap between this and the plate _c_ offers sufficient resistance so that the telephone circuit suffers no leakage at this point, but lightning has such extremely high tension that it readily passes across this gap and finds its way to the earth without damaging the instruments. We have already noticed that our alternating current dynamo, which produces 60 vibrations per second in the telephone receiver, causes it to give a tone very nearly like the _C_, which is two octaves below middle _C_ upon the piano. _C_ requires 64 vibrations per second. We may speed up our dynamo so as to make it yield a tone exactly like _C_ or even above it. Dr. Cahill of Holyoke, Mass., has devised an organ in which alternating current dynamos produce the necessary number of vibrations for each tone. The name _telharmonium_ has been proposed for this organ. It has a separate dynamo for each tone, each dynamo having a frequency corresponding to the tone required of it. The dynamo, for instance, which produces middle _C_ makes the electric currents surge back and forth 256 times a second, and this causes the diaphragm of a telephone receiver to vibrate 256 times a second, and this sends forth 256 air waves per second, and when these reach our ears we recognize the tone we call middle _C_. The frequency of alternation in a dynamo may be increased by either increasing its speed of revolution or by increasing the number of coils upon its armature.