Nikola Tesla Books
Books written by or about Nikola Tesla
Colorado Springs
Nov. 6, 1899
Determination of el. st. capacity of the structure of iron pipes by measurement.
The method previously employed for such a purpose was again used. The coil described on a former occasion, wound on a drum of 10 5/16" and having 550 turns was excited from a vibrating primary system and the maximum resonant rise obtained with only the spark wires attached to the terminals of the coil. Then the upper terminal of the latter was connected to the structure and the maximum rise again obtained. From the two known periods of the primary system and the self-induction of the coil the capacity of the structure was then computed. In order to avoid errors due to the capacity of the wires connected to the coil the precaution was taken to make no change which would in any considerable way affect the result. Thus, when the vibration of the coil with only the spark wires was determined the wire which was to be later connected to the structure was likewise fastened to the upper terminal of the coil in a position such that in the second experiment or series of experiments it was only necessary to tilt the wire to bring it in contact with the structure. If this precaution would not be observed the error introduced by the connecting wire might be - and in fact it would generally be - considerable. The adjustments were first made with the spark wires and the wire which was to be connected to the structure, this wire being placed vertically and at a distance of about four feet from the latter. The maximum rise was observed on the terminals of the excited coil with:
| Capacity in primary circuit total | Turns in regulating coil | Spark analyzing on terminals |
|---|---|---|
| (8x36)-2 / 2 = 143 Bottles or 0.1287 mfd | 2 1/8 | 2 5/8" |
Now the wire was tilted and brought in contact with the structure and resonant maximum rise was observed with:
| Capacity in primary circuit total | Turns in regulating coil | Spark analyzing on terminals |
|---|---|---|
| (8x36)-2 / 2 = 143 Bottles=0.1287 mfd | 17 1/2 | 2 5/8" |
To determine more satisfactorily the self-induction in both the primary vibrations readings were taken in the following manner. The two primary turns, connections to breaks and condensers and the regulating coil were all connected in series - the breaks and condensers being of course bridged by stout and short wires (No. 2 being used) and readings of e.m.f. across the two primary turns were first taken and then across the two primaries plus connections and turns in the regulating coil as were used in the two instances. Since the resistances were entirely negligible with respect to the inductances it was only necessary to make the ratio of the e.m.f. in two instances to determine the inductance of the connections and turns from the known inductance of the two primary turns which was carefully determined before. As the readings were taken practically at the same moments across the primaries alone and across the primaries connections and turns Ï could not vary perceptibly, and to make sure of that the readings were taken repeatedly. The current passed through the inductances also remaining the same during the two consecutive readings, the results ought to be therefore more reliable then those obtained otherwise.
November 6
Tesla carefully measured the capacitance of the aerial pole by the resonance method, from the known inductance of the 550-turn coil (see September 8th) and known frequency of the oscillator, with two measurements, one with and the other without the capacity to be measured. He did not make use of his earlier results for the inductance of the regulating coil and connections from October 30th, although he could have done. A calculation check shows that the results of October 30th were rather high (by as much as 10% for 2 1/8 turns of the regulating coil), but Tesla probably thought that the new procedure was better and so did not use the old results. An analysis shows that if the old values had been used the final result would not have been essentially affected, so that Teslaâs conclusion that the measured capacity of the pole was less than the theoretical value of November 5th remains valid.
From the relatively lengthy discussion following the measurements it may be seen that Tesla expected just the opposite. As usual when his expectations were not fulfilled, he considers ways for getting more reliable results.
November 6
He measures the capacitance of the structure with the sphere by means of the resonant method very carefully, on the basis of known coil inductance of 550 turns (please see Sept. 8) and known oscillator frequency, from two measurements he finds the desired capacitance. The first measurement is without connected structure and the second with it. The resonance he determines on the basis of the maximum spark and the frequency on the basis of inductance and capacitance in the oscillator circuit. In a series of separate measurements he determines the value of a regulating coil as well as the inductance of the connections by following a new procedure.
The primary coil, and the regulating coil with connections as in the measuring circuit is connected to the source of low frequency (most probably as previously 140 Hz) and it measures the voltage ratios shown in the figure. From the previously determined primary coil inductance and the voltage ratio, the total inductance of the connections and regulating coil is obtained. It is interesting that he did not use the results for regulating coil inductance and the inductance of connections obtained on Oct. 30, as he could use them. The calculation check of the previous result indicates that the values obtained on Oct. 30 were somewhat higher (even more than 10% higher for two 2-1/8 regulating coil turns), considered that the newest procedure is better and that is why he did not use the old results. The analysis indicated that even with the old values final results would not change much and Tesla's conclusion is still valid in that the measured value of the structure capacitance is smaller than theoretical value obtained on Nov. 5. From Tesla's relatively long explanation after he performed measurements, it could be seen that he expected exactly the opposite effect. As always so far when his expectations were not met, he continues the considerations about further steps for the purpose of achieving more reliable results.
