The Longines “Ultra-Chron” is a non-magnetic watch, i.e. protected from the influence of ordinary magnetic fields.
If it has been accidentally exposed to an intense magnetic field, the result may be a disturbance of rate, which the watchmaker can easily eliminate by demagnetizing the watch with one of the appliances made for the purpose (the Magnomatic or some other type).
After this treatment, the watchmaker will be obliged to test the daily rate on a liming-machine and, if necessary, to correct it in order to eliminate the residual effect of demagnetization, which may cause a difference of rate of +- 10 seconds per day.
