PHYSICS. 251 



be measured. The latter galvanometer has a coil of low re- 

 sistance, with shunts by which its delicacy may by still fur- 

 ther reduced. Using the galvanometer alone, one division 

 on the scale represents one ten-thousandth of a B. A. unit. 

 With the first shunt the divisions represent hundredths, and 

 with the second whole units. Current strengths from 0.0001 

 to 200 units may thus be measured. These instruments are 

 accurate to one per cent., sufficient for testing currents used 

 in medicine, for which they were devised. 



Foster has exhibited to the London Physical Society a 

 very simple form of the trap-door form of Thomson's abso- 

 lute electrometer. According to Nature, one arm of a bal- 

 ance lias suspended to it by silk fibres a zinc disk hanging 

 horizontally in the plane of a sheet of the same metal, which 

 acts as a guard plate. Below the disk about one inch is a 

 second horizontal sheet of zinc. The guard plate and disk 

 are electrically connected by a bridge of very fine wire. To 

 use the apparatus it is first accurately counterpoised, an ex- 

 cess weight say, of one grain is added, the guard plate 

 and the lower attracting plate are connected with the elec- 

 trodes of the electromotor a Holtz machine, for example a 

 spark-measurer being introduced into the same circuit. If 

 the machine be put in action, and the knobs of the spark- 

 measurer gradually separated, a point will be reached w T here 

 the attraction of the suspended disk just balances the excess 

 weight. Reading off" the length of spark, the data are ob- 

 tained for calculating the difference of potential required. 



Edison described, at the St. Louis meeting of the American 

 Association, a new form of voltameter. Into a suitable ves- 

 sel of acidulated water two electrodes are placed, one of 

 which consists of platinum wire covered with gutta-percha, 

 and perforated with a fine needle near its lower extremity. 

 This electrode is made negative. The evolved hydrogen es- 

 capes in bubbles from the minute opening with a sound like 

 the ticking of a watch, audible at the distance of several 

 feet. By placing a rheostat in circuit, and regulating the 

 bubbles to one a second, a constant current is obtained ; and 

 by calibrating the instrument by this means, the strength of 

 any given current flowing through the instrument is known 

 in terms of the number of gas bubbles evolved per minute. 

 Should this number rise above 16 per second, a musical note 



