416 Mr. T. T. P. B. Warren on a Method of 



in the Astr. Und. (p. 223 et seq.) for isotropic media, to reduce 

 8 to 8 , which value corresponds to the angle X~~^X- The ca ^" 

 culation relative to this, however (which, on account of the dis- 

 symmetry of the expressions that would have to be considered, 

 would become tedious and complicated), we may the rather de- 

 cline to carry out, as the result is absolutely void of practical 

 consequence. 



Not only, therefore, is there nothing to hinder the application 

 of the boundary-equations (II.) even to the perfectly unsymme* 

 trical occurrences at the hinder surface of a plate of glass or 

 crystal in motion (conf. op. cit. pp. 230, 239), but the problem 

 we have treated can itself be so far generalized that the hitherto 

 presupposed coincidence of the plane of incidence with the plane 

 of symmetry can be dispensed with. 



LII. On Warren's Method of finding Faults in Insulated Wires. 

 By Thomas T. P. Bruce Warren, Electrician to Hooper's 

 Telegraph Works, Limited*. 



THE following directions for performing this test are given 

 by Mr. Latimer Clark (Clark and Sabine's Electrical 

 Tables and Formula?). The object of the present paper is to 

 give the more recent methods of employing this test. 



" The coil of wire is wound on two separate drums, both insu- 

 lated, and an electrometer connected to each. A powerful bat- 

 tery is connected to one end of the conductor, the induction and 

 leakage through the dielectric causing each of the electrometers 

 to become deflected. Both drums are now discharged by touch- 

 ing them with the hand, and the electrometers fall to zero. The 

 drum which has a defect on it, however, soon acquires its 

 tension again, and its electrometer is deflected, the other remain- 

 ing unaffected. More wire is then unwound till the fault ap- 

 pears on the other drum. The outside of the wire between the 

 drums must be wiped very dry ; the other parts should be moist." 



This method of performing the test was soon abandoned from 

 its being cumbersome and badly adapted for locating very mi- 

 nute faults, as by the use of two electrometers with different 

 degrees of sensitiveness it was impossible, unless by carefully 

 comparing them, to say on which drum the fault was. Although 

 a galvanometer can be used for the test, an electrometer was 

 preferred, from the fact that the deflected position of the needle 

 or indicator is independent of the length under test, which, by 

 the implied condition of the test, is variable. Obviously with 

 a galvanometer the deflections alone could be but little depended 

 upon. 



* Communicated by the Author. 



