IS opal Institution of (great Britanu 



WEEKLY EVENING MEETING, 



Friday, January 29, 1897. 



Sir James Crichton-Browne, M.D. LL.D. F.R.S. Treasurer 

 and Vice-President, in the Chair. 



Professor Jagadis Chunder Bose, M.A. D.Sc. 

 Professor of Physics in the Presidency College, Calcutta. 



Electro-Magnetic Badiation and the Polarisation of the Electric Bay. 



The great work of Hertz in verifying the anticipations of Maxwell 

 has been followed in this country by many important investigations 

 on Electric Waves. The Royal Institution witnessed the repetition of 

 some of the brilliant experiments of Professors Fitzgerald and Lodge. 

 My interest in the subject, and inspiration for work, are to a great 

 extent derived from the memorable addresses delivered in this hall, 

 and I am glad to have an opportunity to lay before you, at this very 

 same place, an account of some work I have been able to carry out. 



As the subject of ether waves produced by periodic electric disturb- 

 ances is to be dealt with in this lecture, a few models exhibiting the 

 production of material waves by periodic mechanical disturbances may 

 be of interest. A pendulum swings backwards and forwards at regular 

 intervals of time ; so does an elastic spring when bent and suddenly 

 released. These periodic strokes produce waves in the surrounding 

 medium ; the aerial waves striking the ear may, under certain condi- 

 tions, produce the sensation of sound. The necessary condition for 

 audibility is, that the frequency of vibration should lie within certain 

 limits. 



As the air is invisible, we cannot see the waves that are produced. 

 Here is a model in which the medium is thrown into visible waves by 

 the action of periodic disturbances. The beaded string representing 

 the medium is connected at its lower end with a revolving electric 

 motor. The rotation of the motor is periodic; observe how the 

 periodic rotation throws the string into wave forms ; how these waves 

 carry energy from the source to a distant place ; how a suitable re- 

 ceiver, a bell for example, is made to respond. I now produce quicker 

 rotation by sending a stronger current through the motor ; the 

 frequency or pitch is raised, and the waves formed are seen to become 

 shorter. By means of the attached counter, the different frequencies 

 are determined. 



Here is a second model, a spiral spring, attached to which is a 



thin string. As the string is pulled, the spring is strained more and 



more, till the thread suddenly breaks. The spring, suddenly released, 



is seen to oscillate up and down. Electric vibration is produced in 



Vol. XV, (No. 91.) x 



