and Experimental Researches therewith. 51 



tive failures, a perfect induction-balance which is not only 

 exquisitely sensitive and exact, but allows us to obtain direct 

 comparative measures of the force or disturbances produced 

 by the introduction of any metal or conductor. 



The instrument now submitted to the Physical Society 

 consists, 1st, of the new induction-currents balance ; 2nd, a 

 microphone, with a clock as a source of sound ; 3rd, electric 

 sonometer, or absolute sound-measurer, a late invention of my 

 own ; 4th, a receiving telephone and three elements of 

 Daniell's battery. In order to have a perfect induction-cur- 

 rents balance suitable for physical research, all its coils as well 

 as the size and amount of wire should be equal. The primary 

 coils a, a f , and the secondary coils b, b r , should be separated and 

 not superposed. The exterior diameter of the coils is 5^ 

 centims., having an interior vacant circular space of 3 

 centims. ; the depth of this flat coil or spool is 10 millims. 

 Upon this box-wood spool are wound 100 metres of No. 32 

 silk-covered copper wire. I use four of such coils, formed 

 into two pairs, the secondary coil being fixed permanently, or 

 by means of an adjustable slide, at a distance of 5 millims. from 

 its primary ; on the second similar pair there is a fine micro- 

 meter-screw, allowing me to adjust the balance to the degree 

 of perfection required. These two pairs of coils should be 

 placed at a distance not less than one metre from each other, 

 so that no disturbing cause may exist from their proximity. 

 The two primary coils are joined in series to the battery, the 

 circuit also passing through the microphone. In place of the 

 telephone I have sometimes used a magnetic pendulum, the 

 swing or the arc described indicating and measuring the 

 forces. I am at present engaged upon a very sensitive volta- 

 meter, which shall indicate and measure the force of rigid 

 induced currents. The telephone, however, is well adapted as 

 an indicator, but not as a measure of the forces brought into 

 action. For this reason I have joined to this instrument an 

 instrument to which I have given the name of electric sono- 

 meter (PI. V.) This consists of three coils c, d, e, similar to those 

 already described, two of which are placed horizontally at a fixed 

 distance of about 40 centims. apart ; and the communication 

 with battery is so arranged that there are similar but opposing 

 poles in each coil ; between these two there is a coil, d, which 

 can be moved on a marked sliding scale,/, divided into milli- 

 metres, in a line with these two opposing primary coils*. This 

 central coil is the secondary one, and connected by means 

 of a switching key with the telephone in place of the induc- 



* If the coils c and e are of unequal size, the zero of the scale will 

 occupy a position similar to that shown on the Plate. 



E2 



