the Measurement of very small Alternating Currents. 603 
a marked alteration of the change ratio of transformation, by 
reason of resonance, when the transformer is used at certain 
voltages. The small capacity of a high-tension electrostatic 
voltmeter across the high-tension terminals of a transformer, 
will, as shown by the figures in Table Ji1. below, sensibly 
affect the change-ratio. 
The transformer tested was worked at a frequency of 
about 80, and the voltage of the high-tension and low-tension 
terminals measured by electrostatic voltmeters over a certain 
range of voltage, no other load being on the high-tension 
side. 
The change-ratio is far from constant, as shown by the 
figures in Table III., and this is clearly due to the resonance 
produced by the capacity of the high-tension voltmeter. 
Under such circumstances, when brush- “discharges are taking 
place in the interior of the voltmeter it possesses a spurious 
and increased capacity, and lence, in using electrostatic 
voltmeters, a single reading should not be accepted too 
readily as giving the change-ratio of a transformer. 




Tasce III. 
| Transformer Terminal Voltages | 
measured by Transformation 
| Kelvin Electrostatic Voltmeters. Ratio. 
) Frequency =80. 
| 1 aid ke Lt, 
) 16,350 515 | ol Tat 
) 16,650 503 | 33131 
) 16,850 460 | 36°6 : 1 
| 16,900 435 | 38°8 : 1 
| 16,750 415 40°3 : 1 
) 15,000 360 / 416; 1 
) 12,000 303 | 39:6: 1 
) | 


As an instance of the application of this hot-wire ammeter 
in measuring the magnetizing currents of small transformers, 
the following figures may be given :—A transformer of 
1 K.w. size had the hot-wire ammeter above described placed 
in series with its primary circuit, and asa check and test of 
accuracy a known high resistance R of 3450 ohms was also 
put in series with the hot-wire ammeter, and a Kelvin electro- 
static voltmeter was connected to the terminals of this high 
resistance. The following readings were then taken :—The 
voltage V at the primary terminals of the transformer, 
the voltage V’ at the terminals of the high resistance R, 

