‘In order to increase the acuteness of the sounds, one or other of these expedients must be employed. The simplest way is to use a screw which can be tightened or relaxed at pleasure, and which in this manner removes the vibrating disk to a greater or less distance from the electro-magnet. This is the case in M. Froment’s instrument, and by this means he has obtained sounds of extraordinary acuteness, although not unpleasant to the ear. ‘M. Froment has not applied the apparatus to a musical instrument, but it is evident that it would be easy to do so; it would only be necessary to make the notes of a key-board act on metallic levers, of a length corresponding to the position required by the disk for the vibration of different tones. These different levers, resting on the disk, would act as a point of contact, but the point would vary in position, according to the touch. ‘If the current were constant, such an instrument would certainly have many advantages over the pipe instruments which are in use, since the vibration might be prolonged at will in the case of each note, and the sounds would be softer; unfortunately the irregular action of the battery makes it difficult in practice. These kinds of instruments are therefore only used as a means of regulating by ear the force of the battery, a much more convenient regulator than the rheometers, since it is possible to estimate by them the variations of the battery during an experiment without any distraction of the mind.’ In 1856 M. Pétrina, of Prague, invented an analogous arrangement, to which he gave the name of electric harmonica, although, strictly speaking, he had not thought of it as a musical instrument. This is what I have said on the subject in vol. iv. of the second edition of my ‘Exposé des applications de l’Electricité,’ published in 1859:-- ‘The principle of this instrument is similar to that of Neef’s rheotome, in which the hammer is replaced by slender rods, whose vibrations produce a sound. Four of these rods are placed side by side, and when moved by keys, and arrested by levers, produce combined sounds of which the origin may be easily shown.’ It is true that nothing is said in this passage of the capability possessed by these instruments of being played at a distance; but this idea was quite legitimate, and German periodicals assert that it was accomplished by M. Pétrina even before 1856. It was the result of what I said at the outset: ‘that electro-magnetism may come to the aid of certain instruments, such as pianos, organs, &c., _in order to enable them to be played at a distance_,’ and I also pointed out the expedients employed for the purpose, and even for setting them at work, under the influence of a small musical box. I did not, however, ascribe importance to the matter, and it is only by way of historical illustration that I speak of these systems.