Maker, Warren Telechron, Inc., Ashland, Massachusetts, USA. Model Type C, 1926, serial no. 7. Page 6.

Technical specifications and functionality.

The Type C was introduced in 1920. The patent drawings shown below are dated 1922 and given the rarity of any actual examples are what have been used to describe the features of the Type C over the years. There is an error that has been successively reproduced over the years. The production Type C did not incorporate the differential dial shown in the first drawing and never did. Every reference I have come across from the definitive article in the NAWCC Bulletin, August 1991, to various web sites that claim to have expertise on Telechron history have this wrong. In fact, I am surprised that this error was even entertained in the the NAWCC article since there was a very good photo in that same article that showed the center dial was merely a seconds dial; the hand connected directly to the seconds escape wheel; which is the case in my example and not a differential dial as was employed in the Type B. I hope that if my documentation of this clock does nothing else, that it corrects this error.

Henry Warrens patent shown above is what the misconception is based upon. The center, differential dial is prominently shown in the diagram.

My guess is that the patent drawings were intended for a design that would imply some form of control of the generators in central power stations in the few areas of the United States that still supplied DC current and where seeing the differential clearly, precisely   and immediately would be of value. The same function as was used for all other of his master clocks to control generators in stations using alternating current (AC). The concept behind of all Warren's master clocks is the comparison of rate between a mechanical master clock to that of a synchronous motor. The self-starting synchronous motor was  Mr. Warren's most important and famous invention. However, in a DC environment a synchronous motor is useless because the motor needs to 'synchronize' to the frequency found in AC current to determine its speed of rotation, something not found in DC current. In order to get around this problem, a rotary converter is employed, (shown below). A rotary converter is a type of electrical machine, resembling a motor, which acts as a mechanical rectifier or inverter. It was used to convert AC to DC or DC to AC power before the advent of chemical (mercury) or current solid state power rectification. The converter is shown in the lower right hand corner of the second patent drawing labeled 4. In this case the DC current is fed into the converter and AC is generated as output to run the synchronous motor in the master clock, labeled 4 and 15 as well as any slave clocks, labeled 2 and 12 in the first and second drawings, respectively. This clock was used in a 25 cycle environment. This may have been a choice of convenience, perhaps because it was easier to make a rotary converter that generated 25 verses 60 cycles. It makes no difference to the center seconds and lower time dials on the master clock as these are all driven by the mechanical clock.

The synchronous motor and how it interacts with the mechanical clock is well depicted in the first drawing. The motor continuously drives the flexible chain upon which the clock's drive weight is hung. The motor continuously winds the weight up as the clock causes the weight to to unwind downward. Warren's other master clocks the A, B, and E were all designed to operate in an AC environment. So the rate of the electrical input from the power station would directly control the rate of the motor and this rate would be compared to the master mechanical clock through various dial indicators. Here the frequency generated from the rotary converter  determines the rate of the synchronous motor. If the frequency from the converter begins to differ either on the fast or slow side the weight will tend to rise or fall, assuming the rate of the master mechanical clock does not change. In Warren's first drawing this weight is clearly connected to an indicator hand to directly read off the sector dial the difference between the two.

In the second drawing the weight is also allowed to touch an arm that is connected to a switch, in the case of the actual clock it is the mercury switch attached to the tilt table. This switch when closed will cause the field of the rotary converter to be more strongly energized, thus slowing the rotor down; decreasing the frequency and allowing the weight to fall until the contact is broken.  This will occur in ever shorter intervals until the motor is virtually in synch with the master clock. In this way the DC line can be precisely controlled using Warren's AC synchronous motor control and mechanical master clock.

The uppermost dial is a friction clutch that allows the operator to manually adjust the the chain and thus the position of the weight immediately. It is calibrated in seconds with 60 seconds for one turn of the dial. In the patent explanation Mr. Warren explains that this may be necessary in the initial setup of the clock or for use where the slave clocks need to be adjusted for daylight savings time.

From the NAWCC article it is clear that two of the installations that the Type C was used in were not power stations but retail commercial establishments that operated in a DC environment. It may be that the Type C never made it into the DC power generation systems. This may be the reason the differential dial was eliminated in the production Type C. If there was any difference between the slaves and the master clock one could look at the dials to see the difference and thus take corrective action. The differential is not needed in this application. In fact a slave is located directly above the the master clock. The photo above was taken in 1923 and was the complete system supplied to the Bowery Bank in New York City; the other described in the article was to the Plaza Hotel also in NYC. The two rotary converters are clearly shown below the control panel to the left. Judging from the control panel layout it looks like there was a dual redundancy. This would make sense since the master clock contains two motors of which only one can be used at a time and the tilt table contains two switches. In other words if one synchronous motor in the clock should fail, or a rotary converter, or some other electrical component, the system could immediately be switched to the backup. A redundancy feature was also built into The Type A, but not the Type B or Type E.