Euthekfokd. — On the Magnetization of Iron. 495 



the deflection was observed, although resistance of platinoid 

 wire was 130 ohms and of the copper not more than 3 ohms. 

 This shows that the quantity that flows in the secondary is 

 practically independent of the resistance of the conductors. 



This agrees with the theory ; for if M be the coefficient of 

 mutual induction between the two coils, and I self-induction of 

 secondary, then the quantity of electricity circulating in the 



secondary is given by ^— 2 t,2u > and when p is large E may 



be neglected in comparison with 2^L. 



If L = 3,000, and frequency 2,000,000, 

 p = 12 x 10 6 approximately, 

 p"L = 36 . 10 9 . 



Therefore, in order that E 2 may be comparable with p 2 U, E 

 should be 10 ohms or more. 



In the experiments considered, a short carbon rod of 

 10 ohms introduced into the circuit did not alter the effect 

 on the needle, showing that ^L must have been considerably 

 greater in the experiment than in the calculation above. On 

 adding a solenoid of sixty turns in the secondary the fall of 

 deflection was scarcely appreciable, showing that the quantity 

 circulating in the secondary depended on the inductance and 

 not on the resistance of the conductors within wide limits. 



IV. Experiments on the Dumb-bell Oscillator of 



Hertz. 



In the experiments previously considered an ordinary short 

 pianoforte-wire 0032m. in diameter acted very well as a de- 

 tector, but when we come to rates of oscillation of over 

 100,000,000 per second a more delicate detector is required. 



Some very fine steel wire was taken, glass-hard, and cut 

 up into lengths of 1cm. Twenty-four of these little needles 

 were then built up into one, each being first dipped in paraffin 

 to prevent eddy-currents passing from one wire to the other. 

 This little collection of needles formed a compound magnet, 

 and offered considerable surface to the action of rapidly-vary- 

 ing magnetizing forces. The detector was fixed in the end of 

 a thin glass tube for convenience of handling. 



This detector only retained about one-third of its magnet- 

 ism, on account of the demagnetizing influence of its ends. 

 When magnetized and placed in a solenoid of two or three 

 turns it supplied an extremely sensitive means of detecting 

 and measuring oscillatory currents of high frequency. It was 

 far too sensitive to use in the ordinary leyden-jar experiments, 

 for with one turn of wire round the tube it was completely 

 demagnetized by a discharge. 



