April 26, 1889.] 



SCIENCE. 



ii3 



structure, and which carry on their upper ends a pair of brushes 

 which are in contact with two insulated copper bands attached to 

 the circular support of the draw, and moving with it. 



A rheostat is used to regulate the speed of the motor, and a 



easily accessible from the road-bed. The bridge-tender has every 

 thing under complete control, and can easily regulate the speed and 

 the direction of rotation of the drawbridge. 



The motive power is furnished by a 7i-horse-power Thomson- 



Er.ECTRIC DRAWBRIDGE ELEVATION. 



Houston motor, securely fastened to the draw by iron braces. 

 One end of the motor-shaft is a pinion, which drives a train of 

 gears, the last of which turns the shaft formerly operated by the 

 men. 



This installation is complete in every detail. Its operation is 

 excellent, and reflects much credit upon the Thomson-Houston 

 apparatus. The New England Electric Supply Company has re- 

 ceived much praise from mechanical and electrical engineers for 

 the excellent work they have done here, and has applied for patents 

 on the devices used, and is in communication with several cities 

 contemplating installations of the same nature. 



Field\ 



ELECTRIC DRAWBRIDGE PLAN. 



reversing-switch to change the direction of rotation of the arma- 

 ture. The armature, rheostat, and fields are connected in series. 



The double switch, fuses, reversing-switch, and rheostat are en- 

 closed in a water-tight box in the framework of the bridge, and are 



A NEW FORM OF SECOHMMETER. 



At a recent conversazione of the Salters Company in London, a 

 new direct-reading secohmmeter of Professors Ayrton and Perry 

 was shown. This has been designed as a cheaper form than the 

 older instrument, and is intended to be used in comparison of the 

 co-efficients of self and mutual induction. The apparatus, with 

 the cover removed, is shown in the accompanying illustration, taken 

 from the London Electrician. 



In the earlier forms of the secohmmeter, only a make and break 

 were successively made in the battery-circuit, and the circuit of a 

 shunt to the galvanometer; but, by the use of the double commu- 

 tator in the new form of secohmmeter, the sensibility of the ar- 

 rangement is increased fourfold : for, if there be any want of bal- 

 ance in the co-efficients of self or mutual induction that are being 

 compared with one another, or with the capacity of a condenser, 

 the galvanometer receives an impulse in the same direction at every 

 reversal of the battery, which impulse is twice as great, and occurs 

 twice as often, as if the galvanometer-needle received an impulse 

 either only at the making or at the breaking of the battery-circuit, 

 as in the earlier forms of secohmmeter. The fly-wheels make lo 

 revolutions for every revolution of the handle ; and although, by the 

 simple alteration of the gearing previously referred to, the com- 

 mutators can be driven at will, so as to make either two reversals 

 or eight reversals for every revolution of the handle, the ratio of the 

 speed of the fly-wheel to the speed of the driving-handle always re- 

 mains the same, so that the fly-wheel action remains constant. 

 The driving-handle can be conveniently turned by hand at speeds 

 varying from about 60 to 200 revolutions per minute, producing 

 with one arrangement of the gearing 120 to 400 commutations per 

 minute ; so that both the battery and the galvanometer circuits 



