[LB011085], Letter from Thomas Alva Edison to Edward Hibberd Johnson, January 16th, 1882


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[LB011085], Letter from Thomas Alva Edison to Edward Hibberd Johnson, January 16th, 1882

Editor's Notes

We send you by wells Fargo Express (S. S. S[enois]) two candle power Indicators for your Isolated plant or general use if you wish it. They are samples and the same as we use here. They are set to vary at 107 volts your lamps varying from 106 to 108 volts hence by setting them to ring at 107 volts the lower volt lamps will give more than 16 candles and the higher volt less than sixteen candles but the mean will be put about 16 candles.##Now the way we set these Indicators so that they will ring at certain volts is as follows:##We use our standard battery and get a deflection on the Galvanometer in the manner with which Hammer is familiar another being the electromotive force of 1h Dynamo up to the required volts say 107. Then we adjust the spring on the magnet so that it will just ring, decreasing the volts from 107 to 106 which will cause it to stop ringing will putting the volts back to 107 will cause it to ring again. everything is then tightened and it is supposed that it will always be reliable and so far we have had no reason to doubt its reliability Your lamps, which we call London volts are 106-7-8 volts but the standard for ascertaining volts at the Lamp Factory is obtained by Copper Deposit and not with standard battery such as Hammer uses and between the standard battery and the Lamp Co standard you will find a difference if you test several lamps. For instance if Hammer takes a 108 volt lamp put it in the photometer and brings it up to 16 candles and reads the volt s from his galvanometer which he has standardised by his standard Battery he will in all probability find that the 108 volt is wrong because the Lamp Coys standard is a different one. He should try several lamps one after the other and find out the constant difference between the Lamp Coys volts and those obtained from his standard battery. For instance he might find that the Lamp Coys lamps marked 100 volts would show 108 volts by his method and noting this he can readily set the Indicator. We at [Locuck] St use standard battery as you do but when we set these Indicators we of course have to use the Lamp Co standard to arrive at which I have explained above when you get them have Hammer put one in circuit raise the volts until he gets them to 107 (Lamp Co standard)O and then if the bell does not ring you will know the Indicator has got out of adjustment in course of transit Please pay particular attention to this and report fully as we want to know exactly whether shipment affects the Indicators.##We have had one Indicator made in which the bell rang when the candlepower went below 16 candles (i.e. when the volts were reduced) besides ringing on an increase fo candle power. But we do not propose to use this as the sole object of this Indicator is to verify our statement as to the life of the lamps by keeping them at about 16 candles. The parties who use the lamps will be very apt to run them up as close as possible to get the most light out so it would be quite an unnecessary refinement to use this double Indicator as it requires first ajustment and might nulify the whole result and therefore a partial result is much better in practice.##The Indicator is connected up right across the line in multiple arc the same as a lamp. If connected across the line at the machine it will show the candle power of the lamp at the machine and the lamps at the other end of the line will be duller just in proportion to the loss of energy upon your conductors. Perhaps you might place it in about the centre of the line but we prefer to place it at the machine to be on the safe side. Of course when the bell rings all that is required is to work the Resistance Box handle [to] throw its resistance in the field which will bring down the electromotive force and the bell will cease. This should be done a little slowly as it takes a little time for the large magnet which we have to use on these Indicators to alter its magnetism. We obtain reliability by using these large magnets.##In case you have a machine for "B" lamps all you have to do is to short circuit the Terman silver Resistance coils all but that portion around which the Bell is shunted. Then the Indicator will be an Indicator for "B" machines. The resistance of the magnet is the same as that of a B lamp and the resistance of the German silver wire is also the same as a "B" lamp, the two together being equal to a "A" lamp. Hence by short circuiting the resistance you obtain a "B" lamp Indicator.##In our Isolated business we do not send lamps of varying volts but for each customer we give lamps of exactly the same volts Hence the Indicator business becomes very simple.##Francis Jehl Esq is getting along well with the motors and will sail I hope very soon [a for st For.]##Clarke has just told me that the Indicator will ring when with 65 lamps in circuit you turn one off and if you put it on again it will stop.##Your machine is now running (1.15 a.m.) & has been going since 8 p m. The commutator brushes have not been touched since we started. we have been running 940 10 p. E h.p. Those Brushes are immense
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Thomas A. Edison Papers, School of Arts and Sciences, Rutgers University
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