A Galvanometer for Alternate Current Circuits. 309 



the mean depth. This circumstance accounts for the discre- 

 pancies between the numbers in the second and third columns, 

 but would hardly affect at all the comparison in the last two 

 columns. 



The experiments could no doubt be improved upon in 

 various ways, but the agreement, as they stand, between 

 theory and observation seems satisfactory. It may be worth 

 while to remark that a slight inclination of the bed of the 

 tank, or a slight want of symmetry, or even of verticality, in 

 the position of the cylindrical obstacle, would only affect the 

 period by a small quantity of the second order. 



XXVI. A Galvanometer for Alternate Current Circuits. 

 By W. E. Sumpjser, D.Sc, and W. C. S. Phillips, B.Sc* 



THE steady electromotive forces and highly sensitive 

 galvanometers available for use with direct current 

 tests render such tests excellent whether deflexional or 

 balance methods are in question. Tests involving change of 

 current, such as induction measurements, are not so satisfac- 

 tory. In ballistic tests the best galvanometers are in many 

 cases not sensitive enough, and though balance methods are 

 available they are usually not so simple in working as tho?e 

 in which steady currents are employed. Alternate current 

 tests are still less satisfactory. It is impossible to generate 

 an electromotive force whose constancy is comparable with 

 that of a battery or accumulator. Special difficulties arise 

 owing to effects of frequency, wave-form and phase. Balance 

 methods can be devised, but they are rarely of much use, 

 owing to the above difficulties and to the absence of sensitive 

 instruments. A distinct advance has been made during the 

 last few years by the construction of improved forms of 

 vibration galvanometer. But this instrument overcomes only 

 some of the difficulties. It is a sensitive indicator rather 

 than a measuring instrument. It must be adjusted to 

 resonance for the best effects, and its sensitiveness is neces- 

 sarily affected by slight changes in current frequency. 



The sensitiveness of ballistic galvanometer tests can be 

 greatly increased by the use of mechanical commutators such 

 as the secohmmeter of Ayrton and Perry, or subsequent 

 modifications ol this by Fleming and Lyle. But such 

 methods only make use of a crude form of alternate current, 

 and it appears that the simplest and most effective cumulative 

 method for testing effects due to changes of current, must 

 in the end prove to be one involving the use of alternate 

 * Communicated bv the Physical Society : read June 10, 1910. 



