Aug. 1st, 1SS7.] 



SCIENTIFIC NEAVS. 



131 



efficiency of the system now under notice. Instead of 

 being fixed rigidly in the usual way, the dynamo is attached 

 to a cradle, C, and this in turn is swung loosely on the 

 trunnions, T, T, the dynamo being placed in such a position 

 that it and the cradle just balance when the dynamo pulley 

 is removed. This pulley is heavier than an ordinary dy- 

 namo pulley, and when keyed on, its weight gives the 

 necessary tension to the driving belt without causing ex- 

 cessive strains on the bearings. Very smooth driving is 

 thus obtained, with a comparatively low tension on the belt 

 and with little strain on the spindle. 



The arrangement is certainly an ingenious one, and we are 

 glad to hear from Professor Ayrton that it has been tried for 

 some time in a practical way for a portion of the electric light- 

 ing at the Central Institute, and that it has answered very 

 satisfactorily. Among other advantages, it is pointed out 

 that with this system the rim of the fly-wheel need not be 

 specially turned, the axes of rotation of the armature and fly- 

 wheel need not be accurately parallel, and the cradle of the 

 dynamo being low down it can easily be made steady. Rope 

 bands can be used instead of leather belting. 



THE TELEPHONE: ITS PRINCIPLES, 

 CONSTRUCTION, & APPLICATION.— I. 



THE purpose of this paper is to give some description of 

 the telephone from a scientific and practical rather 

 than from a historical point of view. The history of its 

 invention and development is an interesting one, but it 

 bristles with controversial points, and questions of priority 

 of invention, patent rights, and other thorny topics -with 

 which we do not wish to deal here. Therefore, when names 

 of inventors, or dates of inventions are given, they must be 

 taken to indicate simply the persons who introduced the 

 facts to public notice, and the dates at which the facts be- 

 came known to the scientific public. 



To understand the principles and action of the telephone, 

 something must first be known about the laws and nature 

 of sound, and something about the laws of electricity. 



In the first place, it is necessary to remember what sound 

 is before its transmission can be understood. Sound, as 

 known to us, is the result of vibrations or waves falling 

 upon a special organ of sense. Some people, perhaps, have 

 never troubled themselves to consider what kind of vibra- 

 tions or waves sound is caused by. They think vaguely of 

 the waves of the sea, and imagine that waves of some simi- 

 lar kind are formed in the air, or any other medium convey- 

 ing sound. 



These waves or vibrations are simply alternations of high 

 and low pressure in the medium, starting from the source of 

 the sound, and spreading out in a spherical form from that 

 source as a centre, provided that there are no obstacles or 

 reflecting surfaces to deflect or reflect the waves. The old 

 illustration of a stone thrown into a pond is about the best 

 that can be oflered, and the widening circles of crests and 

 troughs, as shown in Fig. i, may be looked upon as a picture, 

 a sectional view so to speak, of the condition of air or any 

 other medium surrounding any source of sound, the crests 

 of the waves being taken to represent compression, and the 

 troughs to represent dilatation of the medium. These waves 

 of pressure acting upon a little circular membrane, the drum 

 or tympanic membrane of the ear, are transmitted to the 

 delicate nervous apparatus of the internal inner ear, and so 

 produce the sensations of sound. Those who wish to go into 

 this very interesting and beautiful subject of wave-motion 

 should read Dr. Tyndall's book on "Sound and Light." 



The next point to notice is that all the varying sounds 

 which assail our ears are distinguished simply by two 

 characters ; firstly, the frequency of the vibrations ; secondly, 



the shape or outline of the waves. Frequency of vibration 

 determines pitch. The shape of the waves determines 

 character, quality, or timbre of the sound. Sounds which are 

 devoid of definite rate of vibration may be classed as noises, 

 while sounds which have a definite vibration-rate may be 

 classed as musical,this definition being ofcourse a scientific, not 

 an artistic one. If two sources of sound act simultaneously 

 upon any medium, the resultant sound is simply that due to 

 the superposition of the two sets of waves. It may be pos- 

 sible to pick out both sounds, one may predominate, and the 

 other only affect its timbre or character, or they may be so 

 blended as to produce the impression of one sound only. 

 The subject is a tempting one, but must not lead us astray 

 from that which we are considering. Enough has been said 

 to show that all sounds are simply due to successive waves 

 of high and low pressure in the medium of their propaga- 

 tion. The succession of waves may be regular or irregular, 

 the waves may be rough or smooth in outline, and every 



Fig. I. — Illustrating Sound Waves. 



difference in either character produces a difference in the 

 sound. It is further to be noted that difference in loudness 

 is due simply to difference of amplitude of the waves, or in 

 other words to the amount of the differences in the pres- 

 sures at the crest and in the trough of the wave. 



From this it may be easily seen that to produce any sound 

 ot a given character and pitch, it is only necessary to give 

 the medium of propagation a succession of impulses at some 

 particular rate, and in some particular order, as regards their 

 relative amount. If the medium is air, or any other fluid, 

 this can be done by causing some flat surface to vibrate in 

 the medium in a manner, as to frequency and other points, 

 corresponding to the sound vibrations required. It will also 

 be obvious that any flat surface exposed to air vibrations, 

 and free to move in any way, may be set into vibrations 

 corresponding to those of the air. As a matter of fact, how- 

 ever, scarcely any solid body, of any shape, is at liberty to 

 vibrate in all ways. Nearly everything, even a body of air 

 in a tube or other vessel, has a strong tendency to vibrate 

 at some particular rate, or rates, and in some particular 

 way, and will only respond freely to vibrations similar in 



