Four Z‐variometers of the knife‐edge type were operated simultaneously at the Cheltenham Magnetic Observatory for six weeks in 1935. Their relative performance was determined by comparing the ordinates for four periods of one hour for each day. The results indicate that variometers of this type, carefully designed and of superior workmanship, can be operated satisfactorily for observatory purposes, if they are kept free from mechanical jars and the supporting surfaces are kept clean.