TEE WORLD'S ADVANCE 



137 



should be a No. 30 bare copper 

 wire, as this will give a full scale 

 reading at 7.5 amperes. The 

 extra holes in each wire holder 

 are to permit of the connection 

 of shunts. The shunt, if it is 

 found necessary, is to consist of 

 single wires of the same size, re- 

 sistance and length as the wire 

 measured. Connections may be 

 made from a short piece of flex- 

 ible lamp cord with terminals or 

 from a piece of copper strip. 



For calibration of this meter, 

 it is advisable to use a direct cur- 

 rent supplied by a storage bat- 

 tery or a number of dry cells 

 connected in multiple. The hot 

 wire meter should be connected 

 in series with a standard direct 

 current instrument and a rheo- 

 stat capable of being adjusted in 

 steps of *4 ampere at a time. 

 In marking the scale, the great- 

 est care should be taken to avoid 

 drafts, as even one's breath on 

 the heated wire will cool it sufficiently to 

 falsify the results. 



This meter will read with equal accu- 

 racy on either alternating or direct cur- 

 rent, and its reading on high frequency 

 current will be the same. The only pre- 

 caution in connection with its use is to 



fig. 5 



Front View of the Working Parts of the Hot Wire Meter as 

 It Appears When Finished. 



make sure that the meter is not over- 

 loaded, as in such an event the wire 

 would quite likely stretch seriously or 

 else burn in two. This would, of course, 

 necessitate replacing the wire and recali- 

 brating the instrument, involving a great 

 deal of extra work. 



A GOOD STRAIN INSULATOR 



Owners of wireless stations always 

 have to make use of insulators in their 

 aerials. Porcelain cleats are generally 

 employed for this purpose, but are found 

 to be useless when they are subjected 

 to any strain such as guy-wires impose 

 upon them. The writer has found that a 

 block of wood with an outer coating of 

 paraffine is an excellent insulator and, 

 moreover, possesses a high tensile 

 strength. The block is dipped in the 

 melted wax and thoroughly impregnated. 

 Wood is naturally a good insulator but 

 has the disadvantage of partially con- 

 ducting an electric current when it is 

 wet, and thus allows considerable leak- 

 age in the case of wireless. The paraffine 



renders the block waterproof, and there- 

 fore makes a good strain insulator. H. 

 C. LOOM is. 



ELIMINATION OF TEST BUZZER 

 NOISE 



The noise made by the test buzzer may 

 be reduced to a minimum by suspending 

 the buzzer from the table edge by means 

 of rubber bands. Furthermore, the con- 

 necting wires running from the table to 

 the buzzer are curled. When the instru- 

 ment is screwed directly to the table, as 

 is usually the case, the table top acts 

 as a sounding board, greatly augmenting 

 the actual sound produced by the buzzer. 

 The same is the case when the buzzer is 

 screwed to a wall. ROYAL BERGVALL. 



