CONNECT TELEPHONES TO MARKED POLARITY

Radio Record, Volume I, Issue 16, 4 November 1927, Page 14

CONNECT TELEPHONES TO MARKED POLARITY

Whenever a pair of high resistance telephones is used in the anode circuit of a valve so that the anode current to the valve passes through them, care should be taken to see that the telephones are connected so that their polarity is correct with respect to tlie

H.T. battery, This point is often overlooked through ignorance, or neglected because considered of no consequence, In actual practice it may greatly affect the sensitivity of a pair of telephones after a time, since a current flowing through the windings in the wrong direction tends to demagnetise the permanent magnets, while a current in the correct direction keeps them magnetised. Just because a new pair of telephones give equally loud signals either way round do not think that this shows that the polarity will not matter ultimately. The sensitivity of telephones depends very largely upon the permanent maguets being powerful. Cases are known where ligh-resistance telephones which had lost their magnetism have been restored and given a new lease of useful life simply by using them connected the right way round in the anode circuit of an amplifier valve taking a fairly large plate current All high-resistauce telephones should have their polarity marked either on the leads or at the terminals of. the earpieces. The telephone lead marked positive should go to the positive terminal of the H.T. battery, while the negative lead should go to the anode of the valve. These remarks, of course, do not ap-| ply to low-resistance telephones used | in conjunction with a telephone transformer. |